If it's bullying, then the victim could be made to look like the perpetrator by the offender. The offender will look for any way to get out of trouble. Not to mention that even if that isn't the case; if the victim starts getting violent without the proper tools to succeed in winning a fight, then the victim could get seriously injured, and/or killed. Even if those two situations don't happen, if there was no proof the victim was being bullied, sometimes just saying "self-defense" won't be enough.
