I have requested an automated way to do this from Meraki, like, every M-F between 8-3, disable iMessage but nothing yet. That would also introduce problems with holidays and summers. So, I have a reminder set to tell me to turn it on and off every weekday and using the Mercury Browser on my iPad a Bookmark Bar link to quickly get in and out. (Mercury is good at saving passwords).
From the settings menu I’ll choose Students and scroll down to iOS Supervised Restrictions. Uncheck Allow iMessage and touch Save. At the end of the day, (3:00 pm), I’ll reverse the process.
Using Mercury Browser on my iPad, this is a 10 second process.