They'd be surprised. The more innocent ones like Tsuyu might just blink in confusion. And Todoroki might raise an eyebrow, not fully understanding what it's all about.
Sakura might react by being more assertive around Naruto. Instead of being shy or holding back, she could be more forward in showing her friendship or even her deeper feelings. She may also try to intimidate the fangirls in a way, like using her strength as a ninja to show that she is not one to be trifled with.
One possible analysis could be that their relationship in fanfic often showcases a strong bond of friendship. Bakugo's explosive personality and Kirishima's more stable and friendly nature create an interesting dynamic. Fanfic writers may explore how they support each other during battles or in their daily lives at school. For example, Kirishima might be the one to calm Bakugo down when he gets too riled up, and Bakugo could inspire Kirishima to be more confident in his abilities.
Kirishima is still alive in the manga. The story doesn't have that plot point. Usually, character deaths are significant events and are clearly shown if they occur.
Naruto might become more competitive. For example, if he sees Hinata spending time with someone else, he'll try to outdo that person in various tasks, like training or helping villagers. He could also get moody and a bit withdrawn, not his usual energetic self. It's his way of dealing with the unfamiliar feeling of jealousy.
Another characteristic is the exploration of his relationships. His friendship with Bakugo is a popular theme. Fanfics might show how they support and challenge each other. Also, his relationships with his other classmates are often explored, like how he can be a source of motivation for them.
Kirishima is from a manga/anime. His back story involves him having a sense of self - doubt initially. He admired those with strong and manly qualities. He worked hard to overcome his insecurities and develop his own strength and courage, eventually becoming a reliable hero - in - training.