If someone hits me first, will I get in trouble for hitting them back? Isn't that just self-defense?

Fighting at school will get you in trouble. It doesn't matter who threw the first punch. When you hit back, you are involved in a fight. If someone wants to fight, there are things you can try to keep yourself out of trouble: get away from the person; find an adult; yell, "stop hitting me!" as loud as you can; remind the other person that fighting has serious consequences; stay calm.