They could react with surprise. After all, Peter is still young. But then they would likely be happy for him. Maybe they'd offer to take MJ out for a nice dinner to get to know her better.
They might be a bit surprised at first. Maybe Tony makes some jokes about British tea and bad teeth, but in a friendly way. Steve would probably be intrigued by Peter's different cultural background and might ask him about British history.
The Avengers finding out could lead to some interesting scenarios. For one, they might want to meet MJ. Natasha could give MJ some self - defense tips in case she ever gets into trouble because of Peter's superhero life. Hulk might not seem too interested at first, but he could end up being the one to make sure Peter doesn't let his relationship affect his superhero duties. And Clint Barton could share some stories about his own family life to give Peter perspective on relationships.
In many fanfictions, the Avengers' reactions vary. Tony Stark might be proud, like he's found a protege. He'd probably start upgrading Peter's suit right away. Captain America, on the other hand, would be more concerned about Peter's moral compass and whether he's ready to be a full - fledged Avenger. Hulk might not care much at first, but once he sees Peter's strength, he could become his ally. Overall, it's a mix of different emotions and actions.
Tony Stark might use his quick wit and technology. He could immediately scan Peter to make sure there's no physical danger causing the hyperventilation. Then he'd probably crack a joke to try and calm Peter down, like 'Kid, you breathe any faster and you'll fly away like a human jetpack.'
Peter might be really nervous at first. He's used to being just the friendly neighborhood Spider - Man, and now the Avengers know he's Tony's son. He could be worried about living up to their expectations.
Thor could be angry. He's a proud warrior and being kicked out like that might be seen as an insult. He might even challenge Tony in some way, either verbally or physically. However, he also has a sense of honor and might listen to Tony's reasons if given the chance.
One way is by getting involved in Peter's school life in a more official capacity. Maybe they could arrange for some anti - bullying seminars at the school. Natasha could also train Peter in stealth and evasion techniques so that he can avoid the bullies more easily. Thor could even offer to be Peter's bodyguard for a while, at least until the bullying situation is resolved.
Natasha Romanoff could show her emotions in a more subtle way. She might look at Peter's old things and a small smile would cross her face as she remembers his innocence and determination. Thor, on the other hand, might become more determined to protect the world in Peter's honor, as he respected Peter's strength despite his young age.
The Avengers would initially be in shock when they find out Peter is sick. Clint Barton would be the one to keep a calm head and start organizing things. Wanda Maximoff would try to use her powers to ease Peter's pain if possible. Vision would use his vast knowledge to try and find a solution. They would all be very concerned and do everything they can to make Peter better, from getting the best medical help to providing a safe and comfortable environment for him to recover.
The Avengers might react in different ways. Steve Rogers would probably be polite but firm in getting to know Peter's boyfriend. He'd want to ensure that he was a good influence on Peter. Thor, on the other hand, might be a bit more boisterous. He could either be really friendly and enthusiastic, or his loud and powerful presence might accidentally scare the boyfriend a bit.