It’s not photoshop but Fotografix but it’s certainly a capable free alternative for the average person.
In addition to being totally portable, it’s really small (less than 1Mb). Despite its size, most of the features I rely on in Photoshop are available. Fotografix supports layers, masks, and scripting, comes with a few key blur and stylization filters, and type layers are editable.
It doesn’t need to be ‘installed’ on a network (just run from a common area).
The usual editing tools are also provided, like rectangular and elliptical marquees, magic wand, rubber stamp, eyedropper, and gradient tool.
The link above is a Geocities site and may be filtered in school. this is an alternative that should still work: