Ruckus runs hot in auto mode. Assuming that there's not more than 10 or so classrooms in the block, I would start with -3 on 5 and -9 on 2.4. Measure the RSSI in a worst case area and verify you're around -60. If you're a lot better than that, cut it some more. You may need to go -6 on 5 and -10 or min on 2.4. If a block contains more than 10 classrooms, you should survey first and see what the RSSI level is in a worst case area.
As for setting the bss minrate, you can do that in the GUI if you're on v10. If the firmware is older, you can only do it via CLI. I use this technique too but not as a first choice. To set the bss minrate you must first be set to ODFM only (no 802.11b rates). To set OFDM only and bss minrate to 12 for a wlan named EXAMPLE, here are the commands:
ruckus> en
ruckus# config
ruckus(config)# wlan EXAMPLE
ruckus(config-wlan)# ofdm-only
ruckus(config-wlan)# bss-minrate 12
ruckus(config-wlan)# exit