Can you recommend an operating system or application for PC time-sharing?

Context:

I work for a small school where I am the academic manager as well as the IT manager.

We’re looking at buying ~20 computers for the use of the students both informally and for ICT-based learning under the supervision of a teacher.

The problem:

Computers are expensive! Computers use lots of electricity! Computers need to be insured!

I’d like to investigate a multi-user timeshare set-up where one CPU is accessed by multiple users on multiple accounts at the same time through separate input devices.

I’ve done some searching and found Userful, but haven’t been able to find anything else that looks even remotely attractive.

So, has anyone used Userful? If so, can you give me details about it including end-user experience and any problems you ran into? If you haven’t used it, have you used anything similar? Can you recommend any other systems like a decent, responsive thin client? Do you know of any other efficient, cheap solutions that I’ve not mentioned?

TIA