If I'm not misunderstanding you I think what you're asking is bordering the realm of Hard AI, where the computer needs to understand the topic that you're discussing.. and were you to solve that slight problem you'd end up not needing the external experts at all as the computer could answer. If you do manage this though let me know and I'll, uhh, market it for you.
Rather than trying to solve this with text analysis I'd recommend you select a selection of internal people who you can trust and who can 'promote' someone up to Expert level based on their perception of their responses. Admittedly a clever troll may be able to baffle them with science, as Magnus Pike used to say, but that's pretty much a problem inherant in the system to be honest.
If you're willing to expose your students to one of the more dangerously time-wasting potential technologies you may want to look into some kind of passworded IRC system for the real time chat. IRC is a stable and mature system with the added benefit that it's got a fairly flexible security system, and that with a few well-placed bots the students have access to data-retrieval and conversation archiving systems. A bot could also be used to restore the priviledged status of the above students should you take that route.
