Yup, I think it should be illegal. There's no reason for a teacher to set up a private social network account with a student that can't be monitored by school authorities. If they need to inform students about homework, projects, etc., they can create a public FB page solely devoted to their class activities, like any normal professional would do for their work.
On the other hand, whether the law is struck down or not is probably gonna depend on other factors besides student safety. For example, it will probably be struck down based on 1A free speech rights or the right to practice a profession (i.e., the right to teach).
See also: http://lawblog.legalmatch.com/2011/08/12/student-teacher-facebook-ban-raises-constitutional-concerns-2/
On the other hand, whether the law is struck down or not is probably gonna depend on other factors besides student safety. For example, it will probably be struck down based on 1A free speech rights or the right to practice a profession (i.e., the right to teach).
See also: http://lawblog.legalmatch.com/2011/08/12/student-teacher-facebook-ban-raises-constitutional-concerns-2/