Or maybe this one if it provided enough oomf...
http://www.ipadcarts.com/solutions/DS-IP-SC-49.htm
All of them are pretty pricey though. Probably better off with multiple Anker USB3 hubs.
That's the 2nd Gen unit of my setup pictured above. It does have enough oomph if you "upgrade" the power supply to something with 35A+ on the 5v. I swapped in an ancient antec on mine.
Their pricing is atrocious. Nothing like fleecing schools.