Bahahaha, school computers can't run any source game, because they always go cheap for a Celeron 2.0 GHz, 512 MB RAM, a cheap battery/power, terrible ventilation and an integrated card. Believe me, I know. At most you can run Halo 1 on low graphics on that
It's better off if you just save the $100 and ask for a computer for Christmas, or go ahead and figure out how to build one yourself.