I can't really argue with you wanting to be on the latest and greatest when it is available. It really comes back to ROI or risk/reward economics. Comparing apples vs apples, a Wave 2 AP will support more 11ac clients, but new features (like MU-MIMO) will limit the number of users that you can support in MU-MIMO groups, which doesn't exist with Wave 1. Keep in mind that the MU-MIMO works one-way, at least in the first iteration, for communication from the AP-to-the-client so your traffic profile will have an effect on how much you will gain from MU-MIMO when the student's devices get there. Also because the first gen of Wave 2 is 4x4:4 the cell sizes are likely smaller vs. 3x3:3 although you can support more density - the reason for the smaller cell sizes is that the total amount of power you can put out is going to be the same whether you are using a 2x2, 3x3, or 4x4 so your density is higher but your coverage is smaller. From a cabling perspective you may want to consider dropping two E/N in the new building, for resiliency in Wave 1 / 2 and for more wired bandwidth, you may push the envelope for Wave 2, but more likely for whatever comes next (802.11ax, etc.) and you want to be using Cat 5e or Cat6 to set you up long-term for 2.5G E/N. You may want to consider Cat6 for 2.5G/5G E/N if you are thinking that far out. The 4x transmitters also means more power so make sure you are going with 802.3at on all your E/N ports. As far as longevity there is plenty of runway on the Wave 1 since we support for 5 years after end-of-sale and we are not anywhere close to EOS of Wave 1. My point here is not to dissuade you one way or another but to give you some information for your planning so that you can decide which is the best for design for your deployment. Hope this helps!
Cheers,
Will