I guess if I were planning for the future, I'd go for Windows 8 as it looks as if it will eventually be a common operating system across all forms of mobile device as well as desktop PCs. The world is also going very much towards the touch interface. In the short term, though, I'd guess many of the 'legacy' portable devices you'll be dealing with will feature a mix of iOS and Android. There's no getting around that, I don't think. It's an interesting strategic conundrum: if you can 'mandate' the pupils' choice of device, then Windows 8 makes a lot of sense. If you can't - you're always going to have a mix of operating environments to support.
Surface Pro (2017) vs Surface Pro 4