I almost forgot to mention Linux. Even with an educator’s discount, you’re still giving a chunk of change to Microsoft if you buy a PC with Windows.
Linux is capable of doing the same stuff Windows is if configured properly, and even the base install includes the basic functions. For instance, most Linux distros come with OpenOffice, so you can handle Word, Excel, and Powerpoint, as well as a PDF reader to handle those now-ubiquitous files, plus media players (video and audio).
A few minutes downloading/installing (free) stuff from the repositories, (pretty much “select what you want, hit the button, and walk away”) and you can get damn near anything else, and the interface is close enough to Windows (even closer with customization) that if you know how to use one, you know enough about the other to at least get by. And if you set up WINE, then you can actually run most Windows programs!
The reason I mention it is because Linux is free. Some computer makers offer certain configurations with Linux for less than the price of the same system running Windows, and if she winds up getting used/donated systems then Linux may be the only cost-effective way to outfit that many computers.
If you saw the sorts of things that are in the repositories, all there for the taking at no cost, you might consider spending a little more on the hardware for better computers since you’ll be saving so much on the software. I am going to guess that, even with the educator’s discount and all, Win7 will cost at least $65 per computer (the “college” price is $64.95 to eligible students, so I imagine the licensing fee for pre-installed systems is comparably priced) and another $60/computer for MS Office (the full, non-trial version, same educational discount). For 18 computers, that is $2250 right there that could have been spent elsewhere. Of course, you won’t actually see half of that cost anyways since it’s in the price of the computer, but that is the difference between “free” and “included”.
Even if you do go with Windows, OpenOffice will still save quite a bit of money while still allowing spreadsheets and powerpoint.