"The school does not allow instructors to cancel a session or write to students."

Where: instructor account: Messages, Cancel session or Restore in Schedule, or Notify students.

Why: the setting "Instructors can cancel a session and write to students" is off (the default), or the owner just turned it off while the screen was open. Without this setting, only the owner can cancel a session and send messages.

What to do: ask the owner to turn the setting on: Settings > Communication > Message settings > "Instructors can cancel a session and write to students", then Save. No need to reload the app: the change arrives when opening Messages, or within 30 seconds when changing screens. Even when on, the setting does not allow editing or deleting a session, or changing the settings.