How do I overcome my fear of seeing people have seizures?

Avoidance. You've trained yourself that this image is so horrible, that you can't survive mentally if you truly witnessed it. Your friends know they survived it and aren't taht affected by it. Your principal was very use to it adn knew what to do (as it seems your mom did too). Best way to stop is to stop running from it (every time you leave the room, you reinforce to yourself that it's too dangerous and scary to be in there). Learn as much as you can about it, watch it in movies and clips online, expose yourself slowly to it until you get use to it and learn how to respond and what is really going on.