If your dog is a real service dog, and task trained to be a seizure detection dog, then that is a different thing. And I highly doubt that you have real PTSD - that is a legitimate use of a specially trained dog. Veterans who have seen horrific things in combat have PTSD - so are some people who have been through a terrible thing. You will not find a dog that is both a seizure detection dog AND a dog for real PTSD. Your parents will need to discuss this with the school administration who are legally allowed to ask what tasks the dog can do for you that you cannot do for yourself. Sounds to me like you want an emotional support animal. They do not have the same access rights that a real service dog does. Leave your pet at home.
And by the way - even if you get your supposed service dog to be allowed in school, there are things that can get the dog kicked out. Barking, showing aggression, urinating/defecating indoors, running around, grabbing food or items from other students - that will get your dog banned from school - legally.