Can an instructor write to students or cancel a class?

Yes, if the owner turns it on. By default, it is not allowed.

1. The owner opens Settings, Communication section, and taps Message settings.
2. They turn on "Instructors can cancel a session and write to students", then tap Save.
3. On the instructor's side, without reloading the app (within 30 seconds, when changing screens): the Messages entry appears, along with the Send a message shortcut on Today and Cancel session / Restore in the schedule.

When on, all instructors of the school can write to any group or to the whole school, see the history, and cancel or restore any session. They still cannot edit or delete a session, change this setting, or see students' contact details.