Currently, if the academic term begins date under Core > Settings > School information > Years & terms has not started. The class won't appear in the “To” field when you go compose the message from your roster.
The system should allow you to message the students for any course during the "School Year" or at least show users a warning message about why the class is not populating in the "To" field.
Teachers need to be able to message students in future courses to give them important information about an upcoming class.