If you're going to punch, keep your fucking thumbs out of your fists so you don't break them.
Don't get your neck within reach of their arms. If they get ahold of it, it's over.
If you have low upper body strength, shin kicks and stomping on people's feet works very well. Figure out what you can do quickly, and do that.
Sometimes, if someone tries to punch you you can sidestep and use their motion to twist their arm behind their back; you can do this with their pinky alone, because people will twist in order to avoid having their pinky broken. It helps if you've had practice. If you can't do the twist, take advantage and jam your thumb into the pressure point an inch or so above their wrist, as hard as you can. Or just bite them somewhere soft, like the trapezial muscles.
There's another pressure point just at the spine, at the base of the shoulder blades. That one is also good for pressing with the thumb, though it's unlikely you'll be able to take their back.
Don't try to knee a girl. It doesn't hurt that much.
Using your elbows is also a good thing to do.
If you can, get a stick. If you can't get a stick easily where you go to school, start carrying a full-size, non-collapsible umbrella.
Many people fight thinking only of the directly forward zone of influence. Small steps to the side can help you to get a good angle, because it's often harder to defend one side of the body than just the front.
Don't give up.
Breath from the tightened stomach into the entire ribcage, and exhale as you strike. Don't do this if you can't do it quickly.
Oh, and ask MaximumZero, not me. He's an actual martial artist, and I'm just a person who has studied a little and ended up in a few difficult situations.
But this is all advice for someone who can't avoid this sort of rough-housing. Inform the authorities and start walking around with groups of friends. At the very least, one of them can run off and tell administration. I recommend including at least one girl, because it's generally assumed that they won't be able to fight and can leave easily.
Discretion is the better part of valor. Don't pick any fights.