Why do people make fun of me becuase of my underwear?

ok every body i go to school and i am very small that means i have a small bird and when i go to the loker room or even to lunch or recess 4 guys come and hang me by my underwear or show every body i went to the change room and i am in grade 8 and i am 4 foot 4 so i am no very tall comparred to every body so i only way weight 7 pounds so i am light what the guys did when i went to the change room is that they stole my pants and my shirt so i was there is my small wight whitities and i had to pee and the other thing and they wouldnt let me go they cept me there for 2 hours and i did it in my underwear so i peed and the other thing in my underwear and then they took pic and one of the guys pulled off my underwear took more pic what do i do i cant beat them up