Dear Zback,
If you don't like your girlfriend, why are you dating her? Just tell her it's absolutely nothing against her, but that you don't think the relationship is right!
Don't ever break-up with a current girlfriend for another girl, it's not right to do that. People would not like that.
But don't worry about dating at this time of your life. Keep focused to school and activities! Dating not important, it can just cause issues and gets in the way of enjoying your single-time before you're taken over with dating (when your older)!