If you prefer parents to use email to correspond with you for individual communication, then you might want to disable the option for them to reply to your whole class announcements.*
You can disable replies from individual family members. Family members whose replies have been disabled will see a note in the message compose box in the Inbox that says ‘Replies have been disabled for this class’.
You need to do this on a family by family basis, there is no option to disable this for a whole class... yet.