<human>: Give me a shortened version of the text

Hello everybody,
So today my girlfriend told me after school that in her power mechanics shop, a guy comes up from behind and grabs both of her butt cheeks. She did it on me to demonstrate what he exactly did and I felt my anger rise to a point I've never felt before. She told him to stop it and he replied that your "boyfriend isn't going to do anything" and "I'll fight him". 

I have the urge to break both of his hands but know that will cause more problems since he is a minor and I'm not. She said that he's done it before and that also pissed me off too because she didn't tell me but I can understand because she was scared. I told her the next time he does it tell the teacher that he's sexually assaulting/harassing her. I would rather take things into my own hands but need to get advice from all of you first before I do. 

I'm not asking for suggestions of how to beat him up but how to take a "speak softly and carry a big stick" mentality. Any advice would be great because I really care about her. Sorry if this was not very smooth but I'm furious and needed to vent. Thanks reddit.
<bot>: GF of six months gets her butt touched by another guy in her class and I need advice on how to handle the situation. Kicking his ass is a last resort. Once again, thanks!