Obviously you want an Ultra-book of some type. They're basically the same as the Macbook air (yes, even aluminium unibodies) but you can get them slightly cheaper (especially in the USA). I don't know which one to recommend on basis of Linux compatibility but I'd suspect most would work well. They also (by Intel requirements) use Intel graphics which pretty much always play nice under Linux - and usually the battery life of an ultrabook is great. In 6 months I would bet that there will be $700 ultrabooks as I can currently find $799 ones.
Distribution wise I would be looking at Fedora (and it's complete ecosystem) or Linux Mint (Debian Edition, MATE and Cinnamon are *great* Gnome 2/3 replacements) rather than the tumultuous *buntu distributions.
The Dell XPS 13 ultrabook apparently going to be shipped with Ubuntu as a "developer" option however the reliability of the Dell consumer range is *terrible* and they also seem to have a $999 price tag. Maybe officially refurbished Macbook Airs are an option as they come with warranty for about $800 and maybe there could be a better education discount.
I would also be consulting with teachers about how/what *they* would want to use whatever hardware. Teachers are mostly concerned about *learning outcomes* and if they don't integrate the laptops (or tablets if you go that route) into the syllabus then they just won't get used.
BTW, on the server side of things you may want to check out Resara Server as an Active Directory replacement - this is more your area right :) ?