As has been said, your an easy target. They are clearly getting something out of bullying you, and its up to you to figure out what it is and remove it from them. Maybe its the satisfaction, in which case, the next time they approach you tell them that your ready and you couldnt really give a **** what they do to you.
Myself have never been very big, but quite fit and not too bad strength wise. Ive only encountered minor bullying twice, and both times i resolved it by snapping. I was considered quite brainy at school, and one kid decided to bait me, i dont know really why it was almost like he was testing me, he would kick me and then step back a minute before moving forward to kick me again while i looked out the window. I considered it that if i didnt do anything i have failed the test and this would continue, for that reason when he did move in the next time i turned around and with another mates technique gave him a jab with my fingers locked straight into his mid section.. he recoiled and left me alone.
The other time i was again being baited by another person, to which i was ignoring as this was later in my eduction, still pre GCSE though.. but had enough of him so just literally snapped and was about to give him a right hook into his face when another person grabbed my arm and dragged me away.. the bully wasnt expecting the reaction at all and i never had a problem again afterwards.