Diagrams on the wall in ICT suites here. Works well.
Permanent marker pen. Each computer is labelled with its location, position and last octet of the IP address eg. Y6001-136. I find that even a marker pen fades with time and I have a little band of helpers who tidy up the suites etc. and they re-mark them when necessary.
We use a little thermal tape labeller. Also use it to label up network points, cabs and cabling. Useful kit.
The kids do peel them sometimes though..the little gits peeled the Dell asset tags off a few and swapped them around last term..played hell with the audit until I realised.
Then we have room plans for all ICT suites in our office for support purposes.
I keep the asset register in Excel and Mail-merge the appropriate information onto small sticky labels (65 to an A4 sheet). These are stuck to the PC & monitor (helped with UHU if necessary) and then covered with clear sticky back plastic.
In a small primary they seem reasonably fiddle proof. We also have "Shut down & switch Off" and "Think before you print" stickers attached in a similar manner to equipment. As machines are numbered in order round the rooms, if one label comes off those of the neighbours will give enough information for me to work out which machine is being reported.
+1 to RoyG.
If a label maker is out of your budget range then a good permanent marker and someone with neat hand-writing!
We use permanent marker - it doesn't last for ever and is a bit of a pain if you have to change the machine number (which seems to happen a lot in this school) but it is OK. Like you, small school, no money!