I'm going out on a limb here because I don't exactly what was posted, but I suspect that in context it was apparent that it was simple name-calling and not an accusation of a crime. If he would've called his teacher a goat fucker, would you really take it seriously? No. Kids at that age try to pick the most hateful thing to say to elicit a reaction. And you know what? Kids of this age already have coping mechanisms in place for when someone calls them a hurtful name.
To be honest, I think this is an overreaction by a vindictive teacher and a staff looking to make an example out of someone, and these kids will suffer a punishment that stretches far into their lifetime for a moment of expressing their frustration to their peers.
This is a rare case where I do think the parents should lawyer up. Don't fuck with a kid's life because you had your feelings hurt when you saw something you were never intended to see.