Right now today in classes kids text, and call each other all the time in class. Give them a Laptop with access to wireless and they will be on IM, Twitter, or facebook. Blocking those sites just means they will take measures to work around it. I wouldn't put it past some enterprising student to build and run their own chat server, basically anytime they are allowed to use it in school they will be doing something you don't want with it. Don't be dumb and end up like that one school district who got in trouble for spying on students.