It's sort of embarrassing, but guys I need help. I don't like how the boys treat me in the locker room ?

I'm 14 and I have a trouble at school. I guess you can say I'm a late bloomer but I don't know. I'm in 8th grade and we all have to get changed in the locker room after class. My mom is sort of weird about what underwear I have...trust me, it is terrible. But regardless, I wear whitey-tighties every day because I don't have the money to buy my own boxers. And the other boys like to walk behind me and slap my butt, or grab me from behind and act like they're humping me hard. It doesn't really hurt, but I just don't like it. I know they're doing it because of my little underwear, because when I sweat they can sort of see my privates. I don't have a way of getting boxers but I don't want to tell any teachers because it's embarrassing. What should I do?!