A class is canceled — does the session still deduct the pack?

No. A canceled session, whether because of a school holiday or because the teacher could not make it, consumes nobody's pack, and any quota already spent is restored.

If the school switches on compensation, students whose session was canceled receive a make-up right, and parents are told through the message channels that the session is off. Deleting a holiday date restores the sessions that were canceled, so a wrong date is not permanent damage.