One of the killers of old equipment is the newer and more bloated operating systems. While hardware gets cycled out of most places every four years or so, the hardware is probably good for eight to ten years. Naturally, keyboards and mouse systems will die before then, but he basic components on the PC will be alive long after you wished that they weren't.
Microsoft has a little-known -- at least I have not heard of it until recently -- version of XP that is designed for older hardware. Only people who have the Microsoft Assurance Program. Usually, this means corporations but school districts may have it too. The operating system is designed to be a platform for the Microsoft remote desktop client. You will not be getting a fully functional Windows XP system. For older hardware, that ship has sailed. But if you have a boat load of older systems, getting this skinny version of XP along with a Windows terminal server could be a good option.
I also like the fact that this system improves your security situation. First, it allows you to run an OS that is being patched. Older systems often come with unsupported operating systems. This new OS puts you back into a supported environment. The second thing that improves security is the limited amount of functionality. Your students are going to have a more difficult time screwing around with computers with this limited OS simply because there is less they can do. With computer security, less is more.