This is the easiest potential solution: just get into a few fights. You don't even have to win them (big plus if you do, though). Just put up a decent showing, and people will soon get the hint that you're not a person to be fvcked with. It really is that simple. Now, if you go to a hardcore school, where getting into a fight can be life threatening, well, this might not be the best advice.
Honestly, I'm amazed at Demon's advice, "be confident". Well, duh. No shyt. How to develop that confidence is what it's all about. Jeezus. You'd think after five years a guy would have learnt something, but nope, he's telling people "be the ball, jimmy".