Tony,
It looks like you've gotten some good advice from people familiar with similar incidents in Oklahoma. Karissa is a neat kid and a good wrestler and she is fortunate to have a supportive father like you to protect her right to wrestle. My advice would be to 1) stand up to the principal and 2) don't hesitate to go over the principal's head to the superintendent. If the superintendent can't see the injustice of barring girls from wrestling, go to the school board. There has to be someone in the school district who can see a Title IX violation in progress and understand the risk such action places on the school district. Hang in there and keep us posted on your progress.