If you're going to write for teens, you should understand at least a little of how/why they think.
Which includes why they do the things they do when they have extreme emotions--and, let's face it, teens have extreme emotions.
Such as the desire to fight.
When boys get into fights, one of two things happens. Either they snap and attack, or they feel threatened and the need to defend. If they snap and are the attacker--watch out. They're not thinking; they're completely physical. But they, often, are only focused on the specific person(s) they want to fight. If you step in between that fight, they (probably) won't (intentionally) hit you, too--boys tend to keep a sense of fighting only the person they want to fight. If they feel the need to defend, you can stop that sort of fight. They don't want to fight, anyway, they feel that they need to.
When girls get into fights, step back. Girls are like rabid, wild lionesses and one should never attempt to stop a girl fight.
Knowing this will make your writing stronger. Think of the different ways boys and girls react to the desire--or need--to fight. Is your character going to snap--and will he inadvertently hurt someone else? Or will your character throw herself into the fight and not care about who they hurt in the way?
(Can you guess what happened in class today?)