If the school gives you loads of really old, worthless pc's you could sell them on ebay, and build a alrightish pc yourself.
My school takes the pc's apart and takes the good parts out of them. They have a huge pile of floppy and cd drives in a cupboard behind the server room. I just asked if I could have one. They were keen to get rid of them. They also have a few old ones for teaching purposes (taking them apart and putting them together again.)