Love the context. A dramatic take on Drillbit Taylor. This could be for this decade what Karate Kid was for the 80s.
To the critique: think logline, not loglines. I don't think this needs two sentences. For the sake of clarity and brevity you ought to sacrifice some information. Really stew on it and distill the essence of the story. I think that articulating directly that he's relocated is wasting words, so can you make that clear through context? Also, as Gabriel said, clear up the "legally forbidden" part. I take it to mean he's on probation because of the sh*t he got up to at his last school but it's always good to be absolutely crystal about these things. And you don't train someone "how" to defend themselves. You just train them to defend themselves. I think this is the kind of logline that'll be best-served by the tried-and-true "When something happens, hero must do something," format, slightly adapted. I'm thinking something like, "When a troubled teen on probation for bullying finds his new school terrorised by bullies far worse than him, he trains a group of outcasts to defend themselves."
Great idea; good luck with it.