MR20 might work, but I think in an environment like that you would want the MR30 minimum, but recommend MR33. The BLE and security radios I think would be beneficial. Also you say 30 devices, which we all know turns out to be like 45-50.
Please do not use an MR20. Without having the scanning radio that the MR33 and above have it will not be able to adapt to RF issues in the environment very well. You have too many users to make this a viable option. You will be forever trying to tweak it to make it work well.
This is really a high density use case. You are going to have not just 30 connections - but 30 simultaneous connections all being used at the same time following a lesson plan.
And lets face it - there is a reasonable chance students will have their phones and other devices attached as well. Even if they aren't meant to.
If you want some future proofing and expect a long life time out of the kit I would take a look at the MR55. It supports WiFi6 (which is a new technology) - so it has a long lifetime ahead of it.
Because most clients are 2x2, a single MR5x series AP can handle about the same load as 2 x MR4x series APs (when using 20Mhz channels). This is because the MR5x series (basically) have twice as many radios in them.
Head this warning - do not go any lower spec. Sure it will work, but you will have intermittent issues around performance and stability.
If it was my money, I would use the MR55. But that's because I like solutions I never have to touch after I put them in.