Nice backchannel tool designed specifically for education.
Backchannel Chat is a class discussion tool that was designed from the ground up to support teachers. Backchannels are great for increasing participation in classroom discussions and for informally assessing your students’ knowledge. Teacher can control all aspects of the discussion, you can remove messages and prevent students from posting, if the pace of the discussion is overwhelming and key messages are lost you can take control and lock the room so that only you can add content. Learn more about Backchannel Chat
Phi Beta Iota: Probably not really secure, and way too hierarchical in its control paradigm, but perhaps a good start.