It's possible that Jackson is someone who runs a fanfiction club or community that Annabeth is part of. The hoodie is a way for her to show her membership or support for that group.
Perhaps Jackson is a friend or acquaintance who writes fanfiction. Annabeth wearing the hoodie could be a show of support for their creative work. It might also be a way to start conversations with others who are also interested in Jackson's fanfiction.
It could be a sign of closeness. Wearing his hoodie might show that Annabeth feels a special connection to Percy, like she wants to be close to him in a very personal way.
Maybe it's about Annabeth taking Percy's hoodie as a sign of closeness. She might wear it around and it makes Percy feel a certain way, like a mix of surprise and a bit of flattery.
One great Percy Jackson hoodie fanfiction is 'The Hoodie's Secret'. It adds a new layer to the story by making the hoodie a magical item that has been passed down through the demigod generations. The author does a great job of describing how the hoodie looks and feels, and it becomes an important part of the main character's journey.
Fanfiction.net is a great place to start. There are a lot of Percy Jackson fanfictions there, and you can use the search function to specifically look for ones that involve the hoodie. Just type in 'Percy Jackson hoodie' and you'll get a list of relevant stories.
Perhaps it's a symbol of their relationship within the fanfiction world. Wearing his hoodie might be her way of showing that she has a special bond with Percy. It could also be that she's cold and the hoodie is just conveniently there, but in the context of a fanfiction, it likely has a deeper meaning related to their relationship.
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.
It could strengthen their relationship. It's a physical manifestation of their closeness and might make them more aware of their feelings for each other.