Being kept behind after school is read as feeling held back or punished for a small misstep. Dream books link it to guilt and to the sense that time is being taken from you.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.