Powerful 1 room setup.

[DavidS]

Use 8 Raspberry Pi boards linked by GPIO to build a simple server, and then 1 RPi each for terminals.  For up to about 30 terminals you would only need 1 ATX power supply for everything (except the displays).  This would be a quite effective system for many applications, including a class room environment.  You could even take it a step further and distribute processing across the terminals as well, giving the multi threaded combined power of up to 38 RPi's per work cluster.

[Warringer]

You'd be better off to use Ethernet to link the Pis, through a PCB would work nicely to get power to the boards.

[DavidS]

The terminal Pis would be linked by Ethernet, though for the main box GPIO (using DMA) can comunicate a good bit faster.  Also this would reduce the cost of the server box as only one board would have to have Ethernet, thus the rest could be $25 Rev A devices.  Should note I do not think that a PCB should be used, rather a bit of cabling.