All Macs come loaded with software ideal to the classroom, iMovie, iPhoto, Garageband and Appleworks to name just the obvious ones.
They are also easy to network, virus free and, as your friend points out never crash.
All of these points make the Mac perfect for a school situation where troubleshooting time is often not available - especially when they already have software licences available.
Add in the cheap price of the new Mini and the existing knowledge on site and I'd clearly recomend that they stick with the Mac.
Of course, if the existing Macs are old G3 iMacs the price of new monitors and keyboards will add to the cost of aquisition, but that cost would incur with PCs too.
You can get more info from Apples education site: http://www.apple.com/education/