X11 isn't the route you want to go with primary school children and teachers. Especially not on ancient PowerPC Macs.
Unfortunately, Mac OS X v10.3 was obsoleted half a decade ago. It's hard to find anything that runs on it still. You might look for a way to acquire an old version of NeoOffice (which I think used Java instead of X11, so I'm not sure it's that much better, or old licenses to MS Office for Mac, AppleWorks, or iWork '06 ('06 was the last version to support 10.3, and that was only 10.3.9, which should be a free upgrade -- run Software Updates if you haven't already). iWork '06 only had Pages and Keynote, no Numbers, so if they need a spreadsheet, that's a deal-breaker.
You can't even get a decent browser to run on them, so you can't even point them at Google Docs or anything.
Overall, I'm not sure this donation will be worth the time-expense and frustration (not just yours, but for the students and teachers that would have to try to keep these dinosaurs running). You might be better off trying to eBay these things and donating the money to the school.
I say these things as a longtime Mac user who has always been close to primary school teachers and has seen a lot of ancient-equipment donations go bad.