There's also the possibility that it's a minion of an ancient evil. The evil entity might want to disrupt the balance among the demigods by targeting one of their key members, like Annabeth. The minion could be under a spell or some form of mind control, doing the bidding of the greater evil without really understanding the consequences.
Maybe it's a human who has somehow discovered the world of demigods and is afraid of their existence. They shoot Annabeth out of fear and misunderstanding. Another option is that it's a rogue demigod who has gone bad and is working against their own kind. This character could have a personal grudge against Annabeth specifically, perhaps because of her intelligence and leadership skills which they envy.
It could be a random enemy, like a rogue monster that manages to catch her off - guard.
It could be an enemy from the underworld. Maybe a powerful demon or a vengeful spirit that has a grudge against her. They might use their dark powers to harm her, like sending a bolt of dark energy her way.
It could be a Sith or a Sith - inspired enemy. They are often the ones causing trouble for the Jedi, and shooting Ahsoka would be a way to strike at the Jedi Order.
There's also a possibility that it's an accident. In the chaos of a battle or a tense situation, a stray bullet from either side could hit Katniss, even if no one actually intended to harm her directly.
Maybe an antagonist who wants to disrupt the group dynamic. If Annabeth is a key part of a group, punching her could cause chaos and confusion among her friends. It could also be someone who has been influenced by dark magic or some sort of mind - control in the fanfiction. This would make them act against their normal nature and punch Annabeth without really having a personal grudge.
She might survive due to her strong will and the help of her friends. Maybe they find a healer or some magical cure in time.
In this fanfiction, perhaps Annabeth getting shot is a turning point for her character. She could start to question her own safety and the safety of those around her. Her friends might go on a hunt for the person who shot her, leading to a lot of action and adventure. It could also lead to some emotional moments between her and her closest friends, like Percy, as they worry about her survival.
Her family could be a great help. If they are aware of the situation, they can step in. Maybe her parents can talk to the school authorities or even directly to the bullies' parents. Family support is always crucial in such situations.
There's a chance it could be a new character introduced in the fanfiction. Maybe someone who has a mysterious past and enters Annabeth's life briefly but leaves a big impact, resulting in her pregnancy.
Percy would probably be an important character. He's Annabeth's close friend and love interest. His reaction to Annabeth getting her period could be interesting, like he tries to be supportive but is a bit clumsy at first.