Whether some is trying to intimidate you, belittle you, bully you, insult you, or hurt you, don't take crap from anyone. Don't ignore it, be strong, be confident in yourself. Let them know that you will not be pushed around.
But the important thing to remember is, you can do this without being insulting, angry, or physical. If someone is glaring at you or showing threatening body language, smile at them, make them uncomfortable. If someone is taunting or shouting at you, tell them that their words only reveal how shallow and insecure they are. Use the truth of their actions against them. If someone is beating you up, or trying to, fight back and scream for someone. It is so important to remember: defending yourself from physical abuse is NOT RESORTING TO VIOLENCE. Not doing anything makes them think that they can continue.
If you are afraid to punch someone who's hurting you for fear of getting, for example, suspended or expelled, remember that it is not worth it to go through life a punching bag just to be on good terms with an establishment that believes the abuser over the victim.
Lastly, love yourself. People who are insecure on the inside and tough on the outside are afraid of those who are confident on both fronts. Don't let someone ruin your life: no choice is a choice, and there are always other choices.