How come this kid at my school was daring to touch my girlfriend without my permission?

I asked this about a few minutes but I guess it's too long so I'll shorten the details for you. My girlfriend was an participant in a fashion show for the school's prom pep rally for this year, she attended the show and there was this kid who sang a Gay Justin Bieber song One Less lonely Girl. He then touched and hold my girlfriend in her waists when the school photographer took the picture in the backstage for the school's newsletter. The whole school saw the newsletter with him holding my girlfriend and I'm mad now. He has tons of girls following him now and i'm afraid he'll take my girlfriend away from me. My girlfriend and I argued and she told me to Chill Out it was just a thing for the school. I wanted to punch this kid's teeth out but I can't do it on school grounds. How come this kid was daring to touch my girlfriend like that ? What could I do about this kid ? I personally think he's gay for his music preferences.