Time to roid up bro
In all seriousness the only things you really can do are you report them to the school or you start taking self defense clashes. You don't need to be strong to be able to defend yourself, on this end I would recommend Brazilian JuJitsu as it was created so that the smaller individual can stand a chance against a larger foe. Of course this implies that you either have money or your parents would give enough of a stuff to pay for self defense classes for you. If your parents don't telling them about the bullying might help but it's a double edged sword where your parents might just contact the principle instead.
I usually would never recommend going online to learn self defense but if all other options are exhausted I SUPPOSE thats better than nothing. The obvious problem being that if you don't have a teacher telling you where you are messing up with your technique then you will be building your skills on a rotten foundation, but for your use case it should be fine, high school kids are paper tigers, they bully you because they know your a pusillanimous individual and wont do anything about it, the minute you drop one of them you earn a MASSIVE amount of respect from the others.
