In your example, “in” is a preposition, so it cannot be used without an object, therefore, we can either leave it out, or use “it” as an object. If a preposition has an object (as required), the preposition will come immediately before its object. On the other hand, in a relative clause, or in a question (with the question word fronted), the preposition can optionally be placed at the end of the phrase: He has a bed in which to sleep / He has a bed to sleep in. To where are you going? / Where are you going to? In modern usage, the preposition usually comes at the end. Finally, when a preposition is used as an adverbial particle, it most certainly can end a sentence: This morning, I slept in. To “sleep in” is a phrasal verb meaning to get up later than usual. In this case, the “preposition” is not really a preposition at all.

Absolutely not. Yesterday there was a sports meeting in our school with a hundred students participating. - Great Yesterday there was a sports meeting in our school with a hundred students participating in. - Nope. Yesterday there was a sports meeting in our school with a hundred students participating in it. - ugly, but maybe strickly legal. Generallay, I'd say "at our school", but "in" is technically correct too.

For this sentence, in is not necessary. If you do want to add in though it would be better to add 'it' after in. So the full sentence could also read 'Yesterday there was a sports meeting in our school with a hundred students participating in it. Hope this helps

No - it’s correct as it is.