A school bag stands for the knowledge and duties you carry from day to day. A heavy one is traditionally read as pressure; a forgotten one as feeling unready.
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.