Well, in fanfiction, his classmates' reactions can vary widely. There are those who are his friends and they would be supportive. They might cover for him when he has to run off to be Spider - Man. On the other hand, there could be classmates who are envious. They might spread rumors about him or try to expose his secrets just out of spite or envy. And then there are the classmates who are just indifferent. They don't really care about Peter's secret life and just see him as an ordinary classmate most of the time.
All Might would be furious. He would see it as his failure to protect Izuku. He would probably mobilize all the resources he has, both in his hero form and his connections in the hero world, to rescue Izuku. And he would be blaming himself all the while for not being there when Izuku was kidnapped.
Some characters might be immediately terrified. They could avoid Izuku or even try to drive him away. For example, classmates who don't understand his quirk might spread rumors about him.
Izuku would probably be terrified at first. After all, he's been kidnapped by villains. But then, his hero - like determination would kick in. He might start looking for ways to escape or at least resist their plans.
Some in 1A might be in awe. For example, if Izuku has a particularly powerful or unique quirk in fanfiction, characters like Mineta might be jealous. He often shows envy towards those with strong abilities.
When Xu Kai's high school classmates evaluated him, they usually said that Xu Kai was very loyal, cheerful, and humorous. In short, based on what he saw in his eyes, he felt that Xu Kai was a real man and was quite outstanding. In addition, Xu Kai's appearance was also very popular among the public. However, some people thought that Xu Kai's acting skills still had room for improvement. Some people thought he was a little pretentious, but there were also others who thought he was handsome and cute. Xu Kai's outstanding performance in the torchbearer mission made Yu Zheng extremely proud. In general, Xu Kai was evaluated as a loyal, cheerful, humorous, outstanding, and good-looking person.
Some classmates might be understanding. They could have been taught to be inclusive and kind, so they don't make a big fuss. However, there are also likely to be some classmates who are curious or even a bit mean. They might tease the child, not fully understanding the situation.
Obviously, he would interact with his younger self. His younger self is a key character as Izuku from the future tries to guide and influence him. Also, All Might is likely to be an important character he interacts with. All Might has a huge influence on Izuku in the original story, and in the past, Izuku could have a different kind of interaction with him, perhaps sharing future knowledge about threats to hero society. His mother would also be someone he interacts with, as she is an important part of his life.