Wireless – Plexiglass and 2.4GHz wifi signal

Assuming a Cisco WLC... Go to "WLAN > Select the WLAN Profile > General Tab > Radio Policy"

Updating my answer to clarify a few things...

• 802.11a operates at a maximum of 54Mbps, only in the 5GHz bands
• 802.11b operates at a maximum of 11Mbps, only in the 2.4GHz bands
• 802.11g operates at a maximum of 54Mbps, only in the 2.4GHz bands

Keep in mind that wireless operates in unlicensed frequency spectrum, so it is a bit like the wild west... Many corporate networks choose to marginalize support for 802.11b/g where possible, due to the rampant use of the 2.4GHz frequency spectrum by consumer devices... today, 5GHz (and thus, 802.11a) tends to have much lower background noise levels, so it is more suitable for business voice and data services... however it also doesn't propagate as well due to the smaller wavelength in 5GHz signals. Use care when you disable 802.11b/g... it might be worth checking for coverage holes, if most of the traffic formerly used 2.4G...

You seem to be going about this the right way (which is the only real question I can find in your post), but I will throw in a few more considerations and/or options.

1. With this many APs deployed and as many users as you expect (and the other non-802.11 devices they bring with such as bluetooth), what is the noise floor in 2.4GHz? It may be that your noise floor is in the -70 to -80 range, which means that "usable" signal outside the room is nominal.
2. You mentioned RF paint (or mesh), behind the APs. Another option to consider is using it on all walls and deploying two APs per room (for redundancy and extra capacity). You may also need to use it on ceilings of rooms below. However, test any product before using it widely as some do not work as advertised.
3. You mention that your transmit power is set to the lowest setting (2dBm) and your antenna is 6dBi. Since you can't turn down the transmit power anymore, you could consider a lower gain antennas or use attenuators to further decrease the signal strength. You will want to make sure that you are able to produce enough signal for a good connection (and high data rates) for a range of devices that will likely be found in the room.
4. Test. Test. Test. High density deployments are a challenge, and the challenges change all the time as technology and the types of devices users bring into the environment are always changing. Unfortunately, you can't just set it in place and walk away.
5. Realize that there will be problems and issues. This is just a fact of life in an environment where you don't have control over devices that will be brought onto your wireless network. Keep a list of device details that have been reported as having issues (type, make, wireless chipset, driver version, OS, what steps resolved the issue, etc) as this will allow you to start finding trends and general solutions so you can provide information (i.e. "try these steps" or "avoid this hardware") to your support staff and/or end users.
6. Subscribe to mailing lists and utilize resources such as forums and industry blogs regularly to keep aware of problems that might be the result of OS updates or other problems. Some examples: Educause wireless-lan mailing list (probably applies since you are an education organization of some sort), Aruba Airheads forums, Ubiquiti forums, Cisco Wireless-Mobility support forums, Ruckus Room (while a number of these are vendor sites, there can be very good information on them and many wireless problems will apply to other vendor environments as well).

Good luck.