Need advice on purchasing (some type of) Mac

[suffragette]

15. I'd also guess most at Snow Leopard with a few on earlier systems

and a few on Lion (though that will increase and may have already started to with new Christmas presents).
Especially with the online classes, you could have older students who might still be using Leopard or students using their parents' systems, which again often isn't the newest one.
With many accessibility options built in, these can vary from OS to OS, sometimes in small ways and sometimes in major ways, and also in the way they interact with other programs, especially some of the online systems.
Text to speech (TTS)has different voices and different ways of accessing them depending on the OS. Alex has been around for awhile now and is pretty good, but apparently in Lion you can access more voices, but have to add them (haven't had a chance to play with Lion yet). Recording text to iTunes as a spoken track changed (in a good way) from being possible with Automator to an easy menu selection.