we started trialling this software called AB Tutor Control
have to say it seems pretty damn amazing and it dont cost and arm and a leg.
it does all the usual vnc type things but you can tile all the monitored workstations across the screen. You can create groups so teachers only monitor their classrooms and dont accidently affect other machines. there's the option to create a classroom plan. you can lock machines with a custom message/graphic. Teachers are well impressed with the ability to remotly use workstations to illustrate things. the same as vnc but the controls are much more user friendly. does other things too.
The teachers are well impressed lol.
and it aint a nightmare to install like vnc is, especially on a network.