Even though Facebook must be used with precaution by teachers, if they link with their pupils/students, to ban it outright becomes silly. It is a matter of education and proper use of a method of communication which can be useful.
“Sex, Drugs… And Facebook? Moral Panic Police Blaming Social Networks For Kids Being Kids” TechDirt, 9 September 2011 – or correlation is not causation
“Is It Stalking To Bombard Someone On Twitter With Offensive Messages?“, TechDirt, 29 August 2011 – the answer is yes, certainly in UK law (public order act)