Blocking iMessage on the school's wireless network?

You will not be able to. The ports iMessage uses are used by other common services so firewall rules will be problemnatic and it cannot be disabled via restrictions. I am hoping that IOS 6 will allow for restrictions for schools and other entities, but for now there is not much you can do other than educate students about proper usage punish those who are not good digital citizens.

You can comment to Apple on the need to include iMessage in the Restrictions via their feedback page:

http://www.apple.com/feedback/ipad.html

The more people who write and provide a solid case for the need for that feature, the more likely it will be that Apple will include it.

Regards.

Not sure what this would do - this url is registered to a Japanese firm???

In mountain lion server, imessages services, you will add those student user profiles or the device profiles to restrict iMessage.

I do not see this as an option in the latest Lion Server, profile manager, IOS restrictions.

It's not an option in Lion. It's an option mentioned in the just-released overview document for Mountain Lion Server, though no details have been given, just that Email, calendar, contacts and messages can all be managed.

Regards.