Why is regular class in session on the last day of school?

If Dexter is old enough to have a job, he is obviously in high school. Therefore, if it's the last day of school, he would still be returning for final exams. But this movie implies that school is completely over. Yes, I know that Dexter's class took a test on the last day of school. But as implied by Mr. Wheat, that was his idea, not a final exam that the whole school was taking. I can understand if Dexter was in junior high or Grammer school. Final exams are not part of the school curriculum at that level of education. Therefore, on the last day of school, the students just have reguiar class and cheer in excitement when the bell rings: summer vacation. But in high school, that is not how it works.