I think the biggest problem with this is most martial arts do NOT teach a person how do street fight. They teach an idealistic pattern of moves that do not easily translate to actual fighting. This was one of the problems Bruce Lee had with the state of martial arts in his day. The Martial arts teacher I had taught us street fighing and one of the moves he describe in a confrontation that he told us about was not very artsy.
The other problem is most kids do not have strenght nor judgement of what to do.
THe best defense would be to RUN.
cheers