Clip in hair extensions in fun colors....clip them in for the day, take them out at the end of the day. Fun on the weekends, no commitment, and you won't get in trouble at school. I also have some hair chalk in fun colors. It washes right out.
I love fun colored hair, and in high school and college I played around with different colors. My hair has been everything from blonde - black - blue - red - purple. I'm lucky that I'm not bald after all of that.