I'm going to answer "custom app" because your IT person should have offered a private LAN and email system to your spec. My bet is you didn't want that and want a custom system.
Here's the situation-
We are considering getting tablets of some kind for ten residents in a drug/alcohol treatment program. We want those ten residents to be able to send messages to their counselors ONLY and to have NO access to the internet at all. We also don't want them to be able to read the other resident's messages. We want the ten tablets to be able to communicate with our main office computer so the counselors can read the messages.
Main computer is an iMac running Mac OS X.
How can I make this happen?