Hey
The translation was a bit unfortunate in this case and the word itself is a bit special
You are right, かける has the meaning of something not being finished/done.
Or to be exact:(from the dictionary)
- to begin (but not complete)/* to be partway doing …
- to be about to(a bit more and…) …
For example:
言いかける - to be about to say; 言いかけ stop in the middle of the sentence etc
作りかける - to be about to make; 作りかけ half made, or to more literally: someone has started making, but has not finished
読みかける - to be about to read; 読みかけ half read, or to more literally: someone has started reading, but has not finished
書きかけ - to be about to write; 書きかけhalf written, or to more literally: someone has started writing, but has not finished
編みかける - to be about to knit(XD); 編みかけhalf knit, or to more literally: someone has started knitting, but has not finished
もう, in this case, means “soon, shortly”.
So this sentence means
"Today’s homework is about to be done(soon).”
But 終わりかけの will sound strange and is not used(but understandable), so instead, if you want to say that something is half finished use やりかけの(someone has started doing something but has not finished it).
やりかけの宿題は犬に喰われた。
My half-finished homework has been eaten by a dog.
(Though I am not sure if it will work on a teacher )
PS
I would like to buy some healing pots