Open Source has a philosophy of developing and sharing software for free use. Some of this software is useful for use in school. Here's some of what we've used:
AndreaMosaic - Make mosaic pictures using your own photographs.
GameMaker - Programming software.
GIMP - The power of Photoshop, for free.
Leonardo Sketch - A vector bast drawing application.
Scratch - Well known programming software.
TUX Paint - Paint for mac.
Sourceforge can provide a place to search for other software. Let use know if there's any other you know of that we've not included.