Your solution is unlikely to be satisfactory to your users. It would take a pretty serious desktop to run six or eight users' desktops, even if you installed six or eight video cards and six or eight different USB mouse/keyboards running at the same time.

I think the closest practical solution similar to what you're suggesting would be a Thin Client setup, where each user would have a low-end computer running off your more powerful server via LAN. You might gain a bit in the way of systems administration, but I don't know how much cheaper this would be than chromebooks, and older hardware that could run Lubuntu on it's own might be your best, fastest, and cheapest approach.