The original mistake is the obvious one.  He went to a party without permission.  The 'what happened', while relevant isn't the issue to me.  

You would hope your child wouldn't take a beating for being part of the bullying group, but sometimes the 'pack mentality' over rides otherwise good judgement.

It's a lesson for the 15 yr old, and maybe for the 13 yr old as well.  What he learned from the lesson probably depends on how much responsibility he took for his part in it.

Thanks for a great post Hope.  Very much appreciated.