Sorry, but other than blocking the ports in the router, there's no way to block iMessages on devices you don't manage. You could block port 5223 which might disable Messages without too collateral damage. Below are listed some other services Apple has on 5223:
XMPP over SSL
Apple Push Notification Service
If you need any of those services to run, then you won't be able to block iMessages. Even if you block it through your WiFi network, that won't stop students with iPhones or iPads with cell service from using it, so if this is a classroom issue rather than one of tying up your WiFi network (in which case Messages really doesn't take up much bandwidth), there's probably little point in blocking Messages on your network even if you can do so.
The ony other option is to require that any student who wishes to use an iOS device in your school allow it to be managed, and that's a policy I think your school would find difficult to implement.
Blocking iMessage would be a contradiction of the BYOD model-- bringing your own device to an institution or organization shouldn't result in this kind of restriction.
Above Studio 1