Bad idea, Lock it down dont let staff do anything either, it will just cause you more issues.
If staff complain tell them its about teh corperate image, when visitors and parents visit the school.
If you allow customisation, you'll end up with a member of staff setting a picture thats not really appropriate such as a holiday snap of a beach with bare breasted ladies in the backgroud (un-noticed by the staff member) then everything kicks off, its not worth it. (and ruins the corporate image of the school with a million different desktops, and makes more work for you to manage loads of GPOs etc)