running arduino.exe on a network client ; cannot write in "..../appdata/"

You could try running the Arduino IDE in portable mode. You do this by creating a folder named portable in the Arduino IDE installation folder and restarting the IDE if it's running, This will cause those files to be located in the portable folder instead of AppData/local/Arduino15.

You are likely to eventually run into problems with XP and ld.exe using any recent version of the Arduino IDE. There is a workaround for this of replacing ld.exe with the file from Arduino IDE 1.0.6. It's probably better to do this right from the start if students will be using Arduino on XP because it's an error that occurs seemingly randomly and will cause the student who encounters it a lot of confusion. Here's the issue report for it:

though there are better explanations of the fix you can find by searching for that error message here on the forum.