ebay!! I'm quite sure if you dont find pc's for grabs, you can buy parts and build machines for under $100 (excluding software); of course your not going to build gaming rigs right? if you have to buy and build as you go, research what the particular machine will be used for and budget accordingly.
other options, check out any local swap meets, salvation army stores, thrift or flee markets in your area. not only that it's for a school right? get involved with the businesses and people in your community and see there's anyone willing to donate for a good cause (the school/kids). ask around and advertise in local newspapers.
people will help for non-profits and schools... believe me, I just came back from Maui hawaii; while their, I actually contributed back to the community (it helps a small business in a big way) all the donated PC's toward schools, public libary's and non-profits.
just get the word out/get involved with your community and people will respond!!