Here is one of several locations where one can get older model computers. Like car prices, old equipment loses value fast. This particular Dell GX150 is being sold for $47. The cost of shipping will come close to the price of purchase.
These units can be useful for schools for a variety of reasons. For terminal clients, these devices are fine. The dedicated terminal clients cost several times more and don't offer significantly more benefits other than small physical size.
Another good use for these old units is to replace a broken computer in a old computer lab. You don't want to get a brand new computer because all the students will fight for the new computer. Getting a unit like the ones you already have makes life simpler.
And, if you use these computers with Linux, these boxes can make great servers.
In an earlier posting I mentioned donated computers. What buying these old computers provide is a bit more control over what you are receiving. Plus, a donation may include a variety of hardware and there are great advantages to having standard hardware.