Kids will be kids
If she stepped on my arm, my natural reaction would have been to push her off but if she just got on the desk to dance, I would have been very annoyed and moved away.
Don't think I would have used as much force either. I wouldn't have been trying to hurt the girl, just get her off. That person seems like they may have been harboring some violent feelings towards the other students and finally found a way to let it out.
Personaliy I don't have an issue with the dancing. It's the end of the school year. My little sisters last day was Friday. Kids like to do crazy things when school is over or almost over. People shouldn't be acting so uppity about this. The only real issue was the girl getting on top of the other persons desk and possibly stepping on their arm.