I'm with @Greenberet .
If you are going to place it in a corner, the MV12N. If you are going to place it on a wall the MV12W.
I would say that 99% of people would be using the MV12N. It's a great all-rounder camera.
If you plan to do weapon detection (knives, guns, etc), facial or emotional recognition, I'd go with the MV22X. If you want to be able to collect evidence for use in court - I'd lean towards the MV22X.
If the classroom is quite large, you might benefit from an MV22 because of its optical zoom.
This MV site planning tool will give you an impression of what to expect.
https://meraki.cisco.com/public/mv-site-survey/index.html