How do I block iMessage on a Network?

I work in a secondary school in Victoria, and we are keen on blocking iMessage and Facetime while connected to the school network. In our context, students own their individual devices and hence we do not want to lock down the individual devices. However, we want to be able to block the appropriate connections such that iMessage would be disabled while at school.

My greatest concern is that blocking the ports outright would cause all sorts of other problems with other browsing and network functions. Any suggestions would be welcomed.