The children cannot finish after 8th period, there is no bus after period 8 on our school...

Question: the children cannot finish the school after 8th period. The have to finish either after 7th or 9th.

Answer:
Please go to menu Specification/Card relationships/Advanced and set the cardrelationship as on this picture:

Now you have entered constraint that when there is a lesson on period 8, there must be also lesson on period 9 (because there must be at least two periods on 8-9, or 8-9 must be empty). So children can't finish on period 8, they have to stay till end of period 9.