This is pretty embarring!?

Basically, a couple of weeks ago a group of about 10 year 7's nicked my sweets. I chased after them, but somehow they'd eaten them all by the time I got there... there were loads. They were pretty scared of me and they paid up, but now everytime i see them in the corridors, they shout stuff at me like asking for sweets and stuff. Now you can understand my embarment when i say i'm in year 11!! But what am i supposed to do about them?? they're all so y when they're around there friends, but when they're alone they're scared of me!! It also amazes me the lack of respect they have for me, when I was that age i would never have dreamed of doing that to a year 11! this sounds pretty pathetic, but it's so frustrating, because if I say anything to them, then they dob me in! thanks!