Bullying among youth is a totally different issue, and shouldn't be treated different than bullying based on any other factor.  Unless it's reaching to the extreme of harassment or cyber harassment (which there are laws against for both adults and children), just send them to the principal's office and tell them to stop being a dick.  No need to get law enforcement involved.

Misgendering someone one time really isn't going to do anything.  Compared to some of the harassment transpeople get, it will be forgotten about in 5 minutes.  What we really need to focus on is the physical attacks, stalking and harassment cases, etc.