Personally, I believe that there are pretty limited situations where social media sites are acceptable to be used in school. I understand a teacher wanting to share what they are doing in their classroom, but they’d have to get permission from all of the students and their parents in order to do so. I can just imagine a situation where a teacher goes to take a picture that they are going to post, and one student’s parent’s said no to letting their child be posted on social media. Can you image how that child might feel when the teacher goes to take a picture with everyone else in the class? That is just one aspect of the situations, though.
One situation I can think of that would be okay is if all the students, who already had social networking accounts, joined a group with the teacher. As long as the teacher is keeping it professional and only posting about classroom things, I think it’d be okay.