[LAU] Thin Client Server/Client Question

Hello All, I am absolutely new to voice communication via IP but have a requirement to setup a teacher-to-student voice communication capability in a thin client system. I am currently using K12LTSP 5EL (Centos5) in a development testbed that will eventually be put into production in a learning lab. I use fl_teachertool to maintain visual contact with the desktop of each connected terminal client (TC). fl_teachertool also allows me to take control (mouse and keyboard and desktop) of any terminal client if I need to demonstrate a task or application to a student. What I would like to be able to do is allow the teacher to communicate with any individual student via headphone and microphone. Also, I need to keep the system completely self-contained. That is, I can not require the system to depend on any type of outside switchboard or voip service provider. Could someone please point me the in proper direction for how to design and build this system? Regards, Murrah Boswell