Here are some great instructions for how to create a low-end Linux sever for under $80. The hardware is $50 and the shipping is $30. Now, because you have a school, you may have a stack of donated computers to figure out. Let's face it, a donation can be a mixed blessing. You risk the wrath of the donor if you toss out the equipment or you end up invest so much time in fixing and maintaining the computer that a new computer would have been less expensive. Using an old computer as a Linux server makes a lot of sense. Linux runs great on low-end equipment. You don't have to use the old keyboard, mouse and monitor. These parts are probably broken, anyway.
The instructions above provide for a fairly general purpose server. If you just want a firewall or email server, you can use even older computers. In the case of the firewall, you don't even need the hard drive.
If you don't have donated computers, you may go to a recycling center (see this posting) or purchase from one of the online resellers (see this posting). And, don't forget that these old units can also be used for terminal server clients.