How do I let students know a class is cancelled because of the weather?

Cancel the session in the schedule: the app immediately offers to notify its students by email.

1. Open Schedule, tap the session, then Cancel session.
2. Write the Reason (optional), for example "Snowstorm", and confirm with Cancel session.
3. The Notify students sheet opens with a pre-filled message (class, date, time, location, reason). Edit it if needed.
4. Tap Preview, check the number of addresses and the students without an address, then Send by email.

The active students of the class's groups are notified (for a child, their family's main contact; a family receives a single email). Emails go out within the next few minutes. Nothing is offered for a session already in the past. An instructor can do this only if the owner turned on "Instructors can cancel a session and write to students" (Settings > Communication > Message settings).