If the kids have USB sticks, load up the portable edition of
Firefox + ePubReader, and they'll be able to take their chosen e-books wherever they go, since ePR stores a copy of each ePub within the Firefox userdata folder, which automatically gets bundled into the Portable Firefox copy.
You'll need to set up some sort of central repository for them to load the books from in the first place, though. An alternative is Lucidor's Lucifox add-on, which lets you store and load the books from a selected external directory, but which is incompatible with ePubReader.
I think that most of the "modern" non-fiction you'll find free online will be likely to be computer programming stuff (or someone's conspiracy-theory screed).
That said, one that might possibly be of interest (and aimed at kids) is Jim Muller's
The Great Logo Adventure, which he offers as a free PDF via the equally free
MSWLogo page. I think I recall it taught basic mathematical and geometry concepts at the same time as the programming, though it may skew a bit young for a high school. See
ACSLogo for a free (and very nifty) MacOS X app if you need one.
For linguistics,
Mark Rosenfelder has some very nice essays up about various aspects of language comparison and usage, and maybe the kids could do projects using the Language Construction Kit, which is fairly popular among conlangers who like to make up their own.
His website is a bit irreverent though, perhaps too much so for the school authorities.
But at least the kids will be able to learn how to say in eight languages handy phrases such as "So far as I can see Heinlein is just a second-rate Ayn Rand.", "If you got flamed it's because you deserved it. ", and "Can we declare this thread officially dead?", giving them realistic practical skills which they'll be able to put to good use in many situations online.
ETA: Also, B&N's NookStudy app comes with a free download offer for 12 of their annotated B&N Classic editions and some PDF SparkCharts, which you can then load onto the school computers.
Not all of them will be to the kids' tastes, but I recall that Alice in Wonderland and Edgar Allan Poe are among them, along with the Scarlet Letter, which seems to be notorious for being assigned reading. They've got little context and influence essays which include movies and such inspired by the classic tales, and might be a more interesting read than the bare bones editions.