Most companies and schools donate them to electronic recycling plants, they take them all apart and process teh individual materials as needed. Often times they reassemble old computers and sell them in bulk or alone at discount prices. Depending on where you live, they may do something different, like donate them to goodwill or something.
Dealing directly with a school for their computers would be rare, and may not happen at all. Check for computer recycling and electronic recycling plants, they will most likely have old computers for sale, and or free.
Also, putting a short ad on craigslist explaining your need for older computers is pretty productive. Ive picked up a few computers for as little as $50 and some for free. These werent the best machines ever, but they serve the purpose of entertaining my young nieces, and cousins.
