Thanks for the request. Currently there is a way to set this property through powershell for groups, but it only works for users in OWA. We have this in our plans for Outlook desktop as well and will be working on it.
How is it June of 2019 and you still haven't fixed this?
Schools very much need this feature. Admin should be able to enter in dates without staff being able to delete/edit.
Would also keep users using your product as they continue their education/adult life. Great idea.