School is read as the place of lessons and comparison. Being back there as an adult is a classic dream, traditionally about being tested or measured again in some area of life. Not finding your classroom points to feeling lost in a new role, while sitting among old classmates is read as revisiting the person you were then and how others saw you. Being unprepared for a lesson is taken as fear that you have missed something everyone else seems to know. A pleasant school dream, oddly, is often read as readiness to learn something new.
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.