re: A Houston H.S. asst Band Director arrested for playing the Skin Flute

So, he was 18...I understand her getting fired for having sex with him, but I don't understand why she was arrested? Obviously it was consentual, and they were both of legal age right? Don't professors/GAs etc. have sex with some of their students often? Once again, I certainly understand why it is grounds for termination at a high school, but I don't understand the legal charges since they were both at least the age of consent.