Until you have to manage a lab full of Windows, then MAC becomes a much better value. With a Mac you don’t have to buy MS office, you get iWork. You also get iMovie, GarageBand and other Apple software that covers almost everything a typical school would need. And you never have to buy updates for either the OS or the other Apple software. You also have less malware to deal with.
Having worked at a school with labs full of Windows machines, the real cost of IT support is not to be discounted.
Maybe a Mac isn’t the best solution for everyone, but in the school environment, they’re great. Most schools have kids doing video projects, presentations, desktop publishing, etc. —- tasks for which an out of the box iMac can handle with aplomb and with minimal hassle or additional expense.
Sure there are probably FOSS alternatives, but if you are the computer lab teacher, the last thing you have time for is managing a lab full of temperamental Linux boxes and the multiple flavors of FOSS software alternatives that frankly, aren’t that good compared to what Mac gives you included. For example, what’s a FOSS version of iMovie that works so powerfully and intuitively?