Hi,
We use iMacs networked with a Mac Pro running OS X Server for managing accounts, locking out preferences/applications, printing quotas, network storage, software distribution, software updates, 'ghosting', drop boxes for the students to submit work to, remote desktop, pulling screens up onto a teacher laptop/plasma screen and quality speakers for demonstrating work, and visa versa for demonstrating techniques to students. For an interface, I went with the MAudio Keystudio 49i (there are bigger versions if you have the room) and they're working really well. Mic and line inputs, and a midi keyboard in one unit, with two phones sockets for working in pairs or plugging a teacher set in as you walk round.