First check with your school. Some have strict rules about teachers and students being Facebook friends on a teacher's personal page. (And we've heard from many teachers who frown upon the practice.) But if the teacher creates a class-wide Facebook page or dedicated Facebook Group, then he or she is most likely using it as a teaching tool. Still, you have the right to investigate further to make sure it's all legit.
Some educators are making use of social learning networks like Edmodo, Schoology, and Collaborize Classroom for their students to discuss issues, collaborate on projects, or just assign and receive homework. Any teacher who requires students to join a social network should send home clear guidelines for how they're using it.
More Back-to-School Questions Asked and Answered