"When school is over" or "after school is over" would both be correct. Usually when we are talking about personally being finished with school (for the semester, the day or permanently) we don't use "the" as the article. If I say "school is over", it is assumed that I'm either talking about my school or someone elses school, depending on the context. The first phrase would refer to something happening as soon you finished school (for the day, semester, etc.). "After school is over" would apply to some time at or following the end of school. You can see that they can both refer to the same time - it is not exact.