I'm a BIG believer is standing up for yourself - I've always been pretty small, and used to defending myself against bullies at school (a long time ago now).
You don't even need to win (but I always did), just go absolutely berserk and they soon give up and pick on someone less 'crazy'.
When I was about 13 years old, I had all the boys from the year above after me, half of the year above that, and even some of my own year. For 3 months or so I couldn't go to dinner, as I would have been caught in the dinner queue - and I had to keep on the move every minute when I wasn't in lessons. During all this time they never managed to catch me with more than two or three older boys, and every time I took out all three and escaped (went berserk again!). Eventually the source of boys chasing me dried up (they didn't like getting hurt), and they convinced a boy in my own class to 'get me', he was a lad called Mitchell, and I didn't like him anyway. He caught me one dinner, with no witnesses - and I beat the sh*t out of him - he didn't get to any lessons that afternoon, he was in sickbay till he went home.
After that they all gave up, and things returned to 'normal'.