Is this for iPads or Macs?

For iPads you can use a configuration profile to block certain apps, although this will be all the time and not just whilst they're on the LAN. If you want it whilst they're only on your LAN I guess you'll have to figure it out using the firewall.

If you have a nat'd ip you can create a smart group and use that to target the devices using a configuration profile. I do the same thing for when they go off our network...I pretty much disable the device

Another option would be to set up Beacons in the school. When a device is in range of the Beacon (i.e. at school), you can apply specific restrictions. When the device goes home, the restrictions profile is removed.

Thanks to all :) I will make some tests with your answers and be back !