According to Mashable:
Inappropriate contact between students and teachers is at the root of the legislation. Senate Bill 54 is designed to protect children from sexual misconduct by teachers, compelling school districts to adopt written policies between teachers and students on electronic media, social networking and other forms of communication.
I guess this makes sense if you don't trust your teachers not to be super-creepy.
But how would anyone know? And is the new rule legal? Can you really tell people who they can and can't be friends with online?