Hi There! Here are some links to Computers that are donated by the US Gov't.. Please understand that the computers they donate are either 486's or Pentium 1's, with a few MAC's also available.
Not all the links work in this article, nor are the links the only sources of Donated CPU's, but it is a start:
Donated CPU's
Note: If you live anywhere a large NASA facility, contact them and get on their list. They do have an active commitment to refurbish the units and reload the software, which is a lot for FREE. These links also offer a TON of free educational (science) materials and opportunities, including having a real Moon Rock sent to your school (Rules/Strict Security issues Apply, but it can be done!). Try:
Main NASA Home page
Scroll down to the bottom of the page. The right drop-down box will link to individual space centers and bases. Search for "Donated Computers"
This is the educational materials link:
NASA FREEBIES
If this info does not help you, repost your requests, including the state in which your School is located. I'll give you state-specific contacts. Thanks!