How can i be more ertive in school?

I'm in a new school (all girls) and I have a very small grade (60 people). I like it, and most people are nice. But there are a few bitches who sort of try to push you around physically and with the way they talk to you. I'm usually outgoing and agreeable. Inside I am very agressive, but don't show it too much unless very provoked. How can I show more ertivness without starting fights?