Students cannot download apps at school

I cannot download apps at school. I am systems administrators at a school in Netherlands with 900 students with an iPad. Ipads are actively used in every days lessons. Our Wifi network is adequately equipped for heavy use, with 75 access points with 4 radios each, Gigabit cabling and network switches. An internetconnectio nof 300 Mbps. The firewall is not blocking the App Store . Still students cannot download apps. Sometimes , when they turn their ipad off and on again, they are able to download one or two apps. Our infrastructure was checked by an external network engineer and he couldn't find any blockage in the firewall or configuration that wouldn't permit communcation with the App Store, except for fragmented packets, which shouldn't be an issue. I have heard that Apple is actively capping bandwidth of users (ip-addresses) of heavy downloaders of the App Store. Is that true?. As we don't have any caching server , that might be the case. I can't however verify that. Do you have any suggestions?