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.
https://meraki.cisco.com/products/wireless/mr55
A slightly cheaper option is the MR53. It supports 802.11ac wave2 - which most of your WiFi clients will probably be right now. It is still a high density AP and will handle the load well.
The MR55 and MR53 will give you a robust problem free solution you never need to touch.
The next step down depends on your clients a bit. If you have lots of Apple Macbook Pro's (or 3x3 clients) then you should take a look at:
MR45: https://meraki.cisco.com/products/wireless/mr45
- A next generation WiFi6 access point
MR42: https://meraki.cisco.com/products/wireless/mr42
- A current generation 802.11ac wave2 access point.
https://meraki.cisco.com/products/wireless/mr53
The vast majority of clients have 2x2 WiFi. So if you are mostly driven by price then take a look at the MR33.
https://meraki.cisco.com/products/wireless/mr33
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.