Percy could be really angry. He might lash out at Annabeth and those involved. He has a fiery temper sometimes, and this would be a huge betrayal.
He could also be very sad and heartbroken. He's deeply in love with Annabeth, so this would be a crushing blow. He might isolate himself from everyone and just wallow in his pain for a while. He might also question all the memories they had together, wondering if they were all lies.
Annabeth could be under some kind of magical influence. In the Percy Jackson universe, there are many magical elements. She could be cursed or charmed without her knowledge. This would be a very interesting take in a fanfiction as it would add an element of mystery and also give her a chance for redemption once the truth is discovered. Also, it could be that she has some internal insecurities that she hasn't dealt with and they manifest in this way, making her act out of character.
She might be a bit shocked at first. But then, being Sally, she'd probably quickly regain her composure and give them a knowing smile. After all, she's seen Percy grow up and she would be happy for him to have such a close relationship with Annabeth.
Well, one possible reason could be that Percy made a really stupid decision without consulting Annabeth first. Like, if he went off on a dangerous mission on his own when they were supposed to be a team. Annabeth is a very strategic person and she might feel betrayed or extremely frustrated by such actions.
It could also be due to some sort of magic or mind - control. A powerful enemy might have cast a spell on Annabeth, forcing her to turn against Percy. In this case, it's not really her own will but the result of external manipulation.
Percy would probably be extremely hurt. He trusts Annabeth deeply, so this betrayal would shatter that trust. He might feel angry at first, wanting to confront both Annabeth and Luke.
One possible reason could be jealousy. Maybe Percy gets more attention or recognition in their fictional world that makes Annabeth envious and thus appear to hate him.
In the fanfiction world, their personalities might be very complementary. Annabeth is intelligent and strategic, while Percy is brave and impulsive. This yin - yang dynamic could lead to a strong attraction, and the want to kiss could be a natural outgrowth of that attraction.
Maybe it's a sign of closeness between them. In many relationships, sharing or wearing each other's clothes can show a deep bond. Annabeth might be cold and Percy's clothes are just there and convenient.
Well, the classmates' reactions could vary a great deal. The more gossip - loving ones would immediately spread rumors that Annabeth and Percy are a couple now. Others, who are friends with them, might just tease Annabeth gently about wearing Percy's hoodie. There could also be some who are completely indifferent as they don't care much about such things. But overall, it would probably create a bit of a stir in the school environment.
Some of Percy's classmates might be in awe of Annabeth. She's smart and confident, so they could be impressed. Others might be a bit jealous that Percy has such an amazing friend.