How do I defend myself when a boy tries to hit me? (I'm a girl)?

My school has pretty violent students around. Sometimes there are students who pick on me because of my hair color or when I stand up for myself when they shove me and sometimes they are male students who are willing to physically harm a female. I'm a very scrawny and weak teenage girl and there's no time to get a teacher to help me when I get into these types of altercations. How do I defend myself if I don't know how to fight?