MMA is not designed for self defense; it's for competition. There are far more appropriate disciplines for self defense. Very few martial arts focus solely on self defense, and those that do are not considered a martial art. For example, Krav Maga is not technically a "martial art". It's a self defense system.
"Self defense" is as much a mindset as it is a skill which means that just about anything can be adapted to self defense. That being said, you'll see people adapt MMA for self defense purposes, but you'll never see people incorporate Krav Maga in a MMA competition. Well, you might, but they would be quickly disqualified; something about groin kicks, eye gouges, and elbow strikes to the throat that just don't add up to a friendly evening of competition.
Personally, I'd recommend trying out a few classes of MMA and a few Karate classes to see which one floats your boat more. I don't see the sense in using something like Krav Maga as a means of self defense among a bunch of children in a high school setting. But that's just me.
Another thing to consider is that one of the most important aspects of self defense is to avoid the fight all together.
Best of luck.