Percy Jackson himself would likely be one of the main characters. Also, Annabeth Chase as she is always by Percy's side. Grover would probably be there too since he's Percy's best friend.
The main characters in a Percy Jackson school assembly fanfiction could be Percy, Annabeth, and Grover for sure. But it could also include some of the new demigods that Percy has met along his journey. They might be there to show the growth of the demigod community. Maybe even some of the minor gods or goddesses could make an appearance and interact with the students during the assembly.
Definitely Percy Jackson, as he is the central figure in the series. Annabeth Chase would be another main character, bringing her intelligence and leadership. Grover Underwood would also be present with his nature - related abilities. Additionally, Nico di Angelo could be there, adding his dark and mysterious element to the assembly. Chiron might also make an appearance as the wise centaur, guiding the demigods and the events of the assembly.
The teachers at the school would be completely clueless. They'd be trying to regain control of the assembly, but when faced with gods, their normal disciplinary methods would be useless. One teacher might try to be brave and tell the gods to leave, only to be turned into a small, squeaking mouse by Hecate as a warning. Meanwhile, Percy and his friends would be scrambling to fix the mess before things got even worse.
In a Percy Jackson school assembly in fanfiction, perhaps the demigods would show up to give a talk. They could share their adventures and warn the students about the dangers of the mythical world. Maybe Percy himself would come up on stage and show off his sword, Riptide, which would cause quite a stir among the students.
Well, in a fanfiction like this, there could be a lot of chaos. Maybe the gods would show up in all their glory during the assembly and start arguing over which cabin at Camp Half - Blood is the best. Percy would probably be in the middle of it, trying to keep the peace.
He would surely interact with Annabeth. Their relationship is central in the Percy Jackson series, so in a Tartarus fanfic, she'd likely be there to support him or be in a similar situation as him.
The main characters would surely include Percy. Then, there could be Tyson, Percy's half - brother. Tyson could be really worried about Percy and use his strength to protect him while he's sick. Also, Chiron in his disguised form as a teacher at the school could play a major role. He has a lot of knowledge about demigod illnesses and could guide Percy's friends on how to help him.
Percy might meet an old enemy who has somehow been resurrected or changed in the future. This enemy could be more powerful and have a new plan to take over the world or destroy the demigods. Or he could meet a group of time - traveling demigods from an even further future who are trying to fix something in Percy's future that has gone wrong.
There's a chance that it could be a character from Percy's family. For example, a distant relative who has been corrupted by some dark magic. They might betray Percy for their own gain or because they are being controlled, and this betrayal could lead to Percy being killed.
He might encounter new demigods who are also on the run or hiding from the traditional demigod society. These could be demigods with different powers or backgrounds that Percy has never seen before.
Some fanfictions might also include new characters specific to the school setting. For example, a mortal friend of Percy's who has always suspected that there was something different about him. When Percy's identity is revealed, this friend could play a crucial role in either helping Percy adjust to the new situation at school or causing some trouble by accidentally spreading the news too widely.
He could meet other demigods from different regions. For example, a demigod from Scandinavia with powers related to ice.