Whether you're of age or underage, if you didn't want it to happen, then it's sexual assault. You should report it to your parents, or a teacher, or the police. If you're scared, then try to be brave and call the police from the bathroom after school or something to report it; that way hopefully no one you know will overhear you, and the perpetrator won't be around either.
It's messed up, and it can be scary, but you have to be strong and understand that it's not your fault and that there are things that exist to protect you from people like your grandfather.
My god, if this is some kind of joke I'm going to be pissed, but part of me kind of hopes it is anyway because assault is terrible and shouldn't happen to anyone, ever.
I wish you good luck,
>>1-san.