I should’ve mentioned this before, but even saying 生徒に話してあげた, while being grammatically correct, sounds weird to me. I can’t explain why, but I think that a better way to say this is 生徒と話をした. I could be wrong about that though, so take that with a grain of salt.
However, the second sentence たくさんの生徒は楽しかった!definitely sounds weird. First, because of the たくさんの生徒 part, which should either be 多くの生徒 or みんな. Again, while たくさんの生徒 is not grammatically incorrect, I believe the word たくさん is usually (but not always) reserved for things, not for people.
The second reason is because of the use of an adjective at the end, which sounds like “A lot of students were fun”. In this case, the verb version of 楽しい, which is 楽しむ, is a better choice.
So, the final sentences should be something like:
昨日は生徒と話をした。多くの生徒は楽しんだ。
HTH!