It seems ironic that before, everyone complained that iOS devices preferred 2.4GHz, and now people are complaining that iOS devices prefer 5GHz 😄
I think the Load Balancing option is by default set to 25% on 2.4GHz, and that's supposed to implement a soft limit (by withholding probe responses?) that tries to steer 25% of clients on to 2.4GHz. I presume if a client desperately tries hard enough, the Ruckus AP's will consider the client in "distress" and allow it to disobey the load balancing anyway.
But 2.4GHz really doesn't have a lot of capacity to begin with in most environments like this. It seems like the ultimate solution will have to involve adding more AP's — 280 iPads on 2 AP's sounds like a lot, even for Ruckus AP's.