Wallwisher is an excellent collaboration tool for students and teachers.

How-To's

No user registration is required to create a wall, but you can create an account.

Look at my wall for more instructions.

 http://www.wallwisher.com/wall/wallwisherworkshop

Please note that the wall page only supports browsers IE7+, Firefox 2+, Safari 3+, Chrome 1+, and Opera 9+