The NComputing devices mentioned earlier could work really nicely for you. The X300 and X550 will let you share one physical computer to multiple users each with their own screen.
Using the non-network version of NComputing's kit graphics performance seems extremely good, and it's not all that bad on the network version either.

Good system imaging may allow you to extend the computer's life beyond five years. If each of the PCs were to boot from network or be automatically reinstalled on a schedule then hardware faults should be quite easy to detect and repair. Wyse have some good software allowing network boot.

Where is the bulk of your budget going? Servers, network switching, or desktops?
I believe Neverfail and DoubleTake both do educational licensing. It might be worth looking at reasonably low cost servers with low cost Enterprise grade H/A rather than high cost servers that are far less likely to fail.

In terms of supplier you should talk to Axess Systems (0800 023 2226), they are one of the UK's top desktop virtualisation suppliers in the Education market and can advise on Citrix, NComputing, VDI, etc.