I know what it is like for many computer teachers in small schools where you do all of the above and then some... I remember getting help desk calls from parents of my students. (Well we were 60kms from the nearest computer store).
So if you are juggling hats as well as budgets there may be a few free tools out there that may be of use to you. Especially if you have admin privileges.
iTalc - I've written about iTalc before - like many tools it can be used for good or evil (webcams recording kids at home - shame on you!!!). iTalc lets you monitor students screens, take them over or even shut them down all from your teacher workstation. Very helpful for working with a teacher who can't seem to connect to a printer.
TightVNC - one of the components of the above italc. TightVNC hooks you into other computers that are not on your desk. I used to use it to control my servers remotely when I was a system admin.
Clonezilla - the open source version of Norton Ghost, where you can set up one computer and roll that setup image to hundreds of others. It also has that magic - redo from start option when you can't figure out how a teacher or students has mucked up a computer so badly that you just have to rebuild it.
The Dude - A free network layout discovery tool. Great for system admins who have a semi tech savvy staff to work with, who occasionally add in new parts of a network without telling you. Also great for that post planning and documentation thing that you have to do sometimes.
OK so these are for the techs. Have you got any other tools in your tool kit? Please let me know.
FOG - Another open source cloning/imaging system that helps you role out a standard operating environment across many computers - recommended by Brian B - check out his blog - This Swiftly Tilting Planet