Use iMessage and Restrictions in iOS
If your tablets are iPads and your Mac runs OS X Mountain Lion, I think you are home-and-hosed!
Give each counsellor and resident separate Apple IDs (no charge). On an iPad, use Settings -> General -> Restictions to turn off access to all Internet services (e.g. Safari) that they cannot have. It's a pretty comprehensive list, but note that iMessage is not part of the list. Secure the restrictions with a passcode only Program Admins know. Then, the conversations can be limited to only counsellor<->resident with no access to any other conversations.
For the Mac side, restrictions are unnecessary if no resident has access to the machine; segregate counsellors by the use of separate accounts if appropriate.