Shelley and Stan's tickets were 851 and 923 (or 975 as briefly shown at first). Later, when they are waiting in line with their tickets in hand, the first digit on both tickets is 8.
When Sharon gives Shelley, Stan and Marvin a ticket, Shelley's ticket says 975. In the next shot the ticket now says 923.
If Monday was day 8, that would make Sunday day 7. Shelly and Stan wouldn't have school on a Sunday.