There are several villains that could be behind it. Kraven the Hunter might be a likely choice as he's always after the most challenging prey, and Peter would be a great target for him. Another could be Doctor Doom if he decided to make a move against the Avengers. He could use pushing Peter off the roof as a way to test the Avengers' defenses. And then there's the Vulture. Even though he had his own story, he could return and, out of jealousy or revenge, push Peter off the roof, especially if he thought Peter was getting too much attention from the Avengers.
One possible villain could be Loki. He's always up to his mischievous ways and might see pushing Peter off the roof as a way to disrupt the Avengers. Another option is Ultron. Even though he was defeated, his remnants could be programmed to target the youngest Avenger. And of course, there's always the Green Goblin. If he somehow made his way into the Avengers universe, he'd be a likely candidate to target Spider - Man.
One possible storyline could be that Peter survives the fall but has to deal with the psychological trauma. He might start to question his place in the Avengers and whether he's really cut out for this superhero life. Another could be that he is saved by an unexpected ally, like Doctor Strange who uses his magic to break his fall. And then there's the idea that his fall sets off a chain of events that makes the Avengers re - evaluate their security and how they protect their youngest member.
One possible plot could be that Peter, feeling overshadowed by the Avengers, decides to play pranks on them as a form of 'bullying'. For example, he might use his web - slinging abilities to move their things around or cause minor annoyances.
It could be Mysterio. He's known for his illusions and deception. He might create false scenarios that make the Avengers think Peter is a threat. He could manipulate the Avengers' perception of Peter's actions, like making it seem like Peter is causing destruction when he's actually trying to help.
One possible plot twist could be that the real danger was not what it seemed at first. For example, while the Avengers thought they were being attacked by a supervillain, it was actually a test set up by a greater power. And Peter discovers this truth while trying to save them. Another twist could be that Peter has to sacrifice one of his powers temporarily to save the Avengers, which changes his approach to being a superhero.
In some fanfictions, Peter could have a secret that he's keeping from the Avengers. When it's eventually revealed, they might feel betrayed and turn against him. It could be something related to his powers, or his past associations that the Avengers find unacceptable, thus resulting in their hatred.
The plot could twist when it turns out that the real danger the Avengers are in was set up by someone close to Peter. It could be a friend or a teacher who has been secretly working for a super - villain. Peter has to overcome the shock of this discovery and still find a way to save the Avengers.
One possible reason could be a misunderstanding. Maybe Peter accidentally gets involved in something that looks bad to the Avengers, like being in the wrong place at the wrong time during a security breach. So they misjudge him and are mean as a result.
Perhaps they call Peter in class because they've discovered some sort of conspiracy that involves the school or someone in it. Peter, being a friendly neighborhood Spider - Man, has a good sense of what's normal and what's not in his neighborhood which includes the school. They might need his help to investigate something strange going on within the school walls.
Maybe Peter was in some kind of trouble at school and they needed to rescue him.
Peter is likely to be in awe. He might be a bit nervous at first, but also extremely excited. He has looked up to the Avengers for a long time, so meeting them would be like a dream come true. He would probably be stuttering a bit as he tries to introduce himself and show his respect.