So I found this sentence with no ending in a book:
高校生になってもその習慣は続き、学校から帰ってくると着替えもせずに制服のまま洗面所に直行。
It’s surrounded by sentences with polite expressions so I would have to guess this is considered formal register also. Either it is a noun without です or a verb without しました.
What could this be?
nominal sentence?
That’s my rather not helpful guess. Maybe you with more knowledge will understand something with this as a hint 
Edit：
Unless it is some rare piece of grammar, you will probably need provide our experts with some context, like a few sentences before and after.
With my little understanding it look like, let’s call it, は-sentence. Just a topic for next sentences. I would take it this way, but I obviously am very likely to be wrong 
Could be, but that doesn’t explain why there’s no だ or です attached.
だ would make it into copula statement, isn’t it? Nominal sentence don’t work like that. Example from English:
“Harry Potter”
That’s nominal sentence. No subject no verb. But you can use as if it is normal sentence sometimes (hence “nominal”):
“What is your favourite book?”
“Harry Potter” meaning “It is Harry Potter”
I edited my first post. dunno if you see it
Yes, I misread the initial post. Probably need some sleep. 
I’m just wondering about the conditions where this can be applied. This is the first time I have seen this in a formal document. Ofc, calling thing out without a verb in conversation would make more sense.
There are nominal sentences ended with こと in japanese. but it is first time I see something looking like another kind of nominal sentence. But it is hard to make nominal sentence in japanese. だ is implied in natural way.
I don’t know if it of any help to you. That just my wild guess I thought it may provide you with some hints.
I don’t know myself though xD
My “Harry Potter” example does not really work in context of Japanese since that would assume copula form as だ is implied. Something more like:
“Making your bed, cleaning after yourself, being polite to others. This is what we expect of you at your age.”
First one is nominal sentence in pure form. In this situation is creating context for next sentence. looks like something similar happens in your sentence. What is strange with my lvl of understanding is lack of こと.
The primary verb appears to be 直行, but they omitted the する for some reason. Maybe it’s an idiom related to 直行? Or perhaps just a stylistic sentence fragment. <-- Like this last sentence. <-- And that one too, as well as this one.
For me it’s just a stylistic sentence that is a monologue. Monologues in books often omit stuff if it’s obvious, I can do the same with English.
Even after becoming a highschooler that custom continued. After coming home from school, without getting changed from my uniform, bathroom straight away.
Edit - just to make things easier for the future. Don’t think of する verbs as verbs. する is a verb, the thing attached to it is just a noun, always.
So it’s looking to me like my wild guess was somehow correct from a purely grammatical perspective… 
I would be grateful if you would not mind providing me with more insight on the matter. I was for a long time under the impression that nominal sentences are not possible in the strict sense in Japanese since the listener will always assume that the subject is implied and it is a copula sentence at heart. But then I discovered こと kind of nominal sentences.
So I would like to ask if I get the difference right between those two kinds of nominal sentences. Let’s say I want to say “no entry” in Japanese:
こと form: 入らないこと。(Literally it is I guess: Act of not entering - > no entry)
This new form: 立ち入り禁止。
So in first kind we transform normal verb sentence into こと in order to not allow listener to assume there is subject. In second we can assume if the context is clear enough that listener will not assign to it an implied だ and a subject (for example when sentence is surrounded by polite forms)?
Please let me know if I got it right or if I am under the spell of confirmation bias xD
(If I sound weird it is because I feel weird… I try to be more British to avoid uncalled for conflict… Feels strange as hell though. )
こと and の nominalize verbs, but する verbs were nouns before they were verbs. So they never needed こと to be nominalized. Your example with 入らない is different than a する verb, because it is a verb that is now an i-adjective, and thus needs nominalization (to create the noun nuance).
I think I got it now. Thanks
Edit:
… It was all the time in the name and I did not even notice. That makes の much clear as well. If english had kanji i would notice it in 5 second xD. I was starting to consider こと having 2 function, but is one and the same function in different situation…
I maybe should pay more attention to nomenclature. Can we have Polish version of BunPro please?
I believe @Asher has it right: it is just a simple omission of the verb. Given that the surrounding sentences use the formal register, if a verb were to be included, it would probably be します.
However, I think that the omission was necessary to convey nuance that wouldn’t be conveyed if the verb were present. I may be wrong, but compared to including the verb, ending it in a VN (verb noun / する noun) without the する gives it the impression that it’s much more directly observable. Not including the verb paints, at least in my mind, a much more tangible experience for the reader, and perhaps even conveys more of a sense that this habit is unchanging.
So while the passage is formal, in cases where using the proper register defeats the ability to convey certain nuance, it is often the author’s feelings and desired impressions to convey that take precedence over formality. (In fact, it can even take precedence over proper tense, as I discovered in an earlier post that prompted me to ask native speakers on HiNative.)
Edit
I apologise anybody wanting to read for the spelling errors and grammar mistakes since it was quite vivid discussion and I have no time to correct it. It all makes sense though so should be not such a problem.
End of edit
I believe we can all agree it is style thing. Nominal sentences are used mostly for that reason in English and even more so in Polish (and Latin if I remember correctly. It was ages ago when I was learning it so it is subject to disbelieve).
But there is no reason to not think about it as jet another legal omission if term “nominal” does not hold much meaning to you (it does to me since is common in 2 language I formally studied: Polish and English). Looking for subject may be tricky but there is not reason to not think it is just “they” as in english “Global warming is getting worse and worse. They are going to destroy the planet!” Who are “they”? Nobody knows, nobody cares. That is not the point of that sentence.
In fact that is how Cure Dolly explains this things. For me it is more understandable to thing about it as normal nominal sentence. Maybe I will be proven wrong late
Btw: I don’t think I am at odds with @Asher here. Different terminology only. Notice his translation is nominal sentence
There are cases where the fact that the noun CAN take する (even if する is not present) has a direct impact on the meaning.
Consider the grammar point N次第. (N is a stand-in for some noun.)
In that case, it usually means “depending on N” (in which case it would have to be followed by で as in N次第で、。。。). However, it can also mean “as soon as,” in which case the N has to be a する noun (and should not be followed by で).
Ignoring the intricacies with the particle で, the meaning is partly conveyed by whether the noun conveys a sense of action on its own. I believe that in Japanese, there are many cases like this, where the noun inherently conveys action just by being a する noun.
EDIT: I’ll include an example from the DoIJG (Dictionary of Intermediate Japanese Grammar) that I think will help drive the point home:
東大を卒業次第、京大の大学院に入学するつもりだ。(Upon graduating from the University of Tokyo, I intend to enter Kyoto University’s graduate school.)
Aside from the fact that で is not present after 次第, one reason why we know that this means “upon graduating/after graduating” as opposed to “depending on whether I graduate” is because the noun, 卒業 can take する. If it could not take する, then N次第 would never convey the “as soon as” meaning.
That a biiig shocker to me actually. Game changer even. I come from (although useful) misunderstanding about how uncommon nominal sentence are, to opinion they are more common in Japanese than in Polish 0_o.
But I will be not able to be part of this disscusion. It going to the terrytory when I am not able to make any “meaningful” gueasses. You will need to give me a “little bit” of time to catch up xD 2N gramma is way beyond my scope.
It is nominal sentence though. So that supports my claim
This is exactly what I had on mind saying “は(like)-sentence”.
So it is exactly as @Asher said: it case of する noun you don’t have to use nominalizers to nominalize the sentence. just drop する. (or even can’t use them I think)
I think we do agree. 
The illusion that we don’t should be attributed to difference in nomenclature we use. I would not think much of it, unless you believe I got something wrong (it happens quite often…). Then please feel free to correct me.
I got where is the confusion, I think! 
As somebody who is not an outsider to English you can be under the impression that this is “verb sentence” because you guys for some reason nominalize verb in the same way you add progressive aspect. But in that case “ing” = こと. It is not a verb. It is a noun created from a verb.
Hope that clears this misunderstanding up
There are a ton of cases in Japanese where instead of a verb, it is a noun. (I wrote the previous sentence in a Japanese way; it is not natural in English. If I were to rewrite it to make it natural in English, it would have a verb at the end, like this: “…where instead of a verb, a noun is used.”)
If this is what you mean by nominalization, then sure. But I just wanted to be specific that the OP’s specific example seems to be one where a verb would be permissible, but they chose to omit it to convey certain nuance.
I am just not fully on board with calling it nominalization because it implies turning something that isn’t a noun into a noun. This is a case where it’s simply a noun. But hey, if you understand it as a nominalized sentence by virtue of the fact that they are not including a verb that they could have included, then go right ahead if that understanding helps you.
EDIT: I regret bringing this up. It really doesn’t matter for the purposes of this discussion and is really off topic. The main deal is that there is no verb, there could have been a verb, but the omission is probably to convey certain nuance.
