If teachers will do nothing about this and they begin to target you then you realy do need to be agresive. you said yourself they target the weaker ones so my guess is if you start knocking a few of them around they will see that you can fight and then maybe they wont be so keen to try anything physical with you (or your friends).
Certainly dont fight them alone (unless ofcourse they corner you while your alone) try get a few of your stronger friends to help you, maybe they wont do anything if there are a few of you. But if you are singled out and they beat you i sudgest calling the police. If you feel thats its realy nescasary to fight them (im not saying doing so would be a good idea) but a weapon could even the odds a little, i wouldnt sudgest a knife or anything because you dont really want to go to jail but some mace might do the trick.

Chris