Im being bullied please help?

today at lunch time and break, my friends have been throwing mud at me, and throwing tomatoes at me in school, they have been saying i look like a troll, when i dont, people have said im agood looking lad. i dont know what i did to them, i think their trying to get me back for something. at school they have been saying i like feet to people, that i like to lick them but ewww i hate feet but they just wont stop and now everyone in my secondary school thinks i have a foot . i have recently started to feel much happier and confident in myself lately but when my friends see im happy they try and make my confidence go back down to their level, i dont know what to do, im on my last year of school then i leave. when they throw stones at me and mud at me i throw it back and then they cant take it and then all gang up on me and call me abusive names, i told my head of year about this, she said she is going to sort it out but i think their going to say that i gred on them!!!!! with 6 weeks to go till i leave secondary school, how am i going to find new friends, this needs to stop because its getting out of hand :( btw im not a woos i just dont want to get expelled if i hit them, because ive been there and done that when i stick up for myself. thanks for your help!!!!!!!!!!!!