To see or use a locker in your dream, signifies aspects of yourself which you have kept hidden inside. Consider what items and belongings are in the locker. In particular, to dream of a school locker, denotes hidden feelings, knowledge, and attitudes that you need to learn and/or acknowledge.
To dream that you cannot open a locker or that your forgot the combination, suggests that you are unsure of where you stand in a particular situation. You feel that you are on shaky ground. If you cannot find your locker, then it also symbolizes your insecurities about your role or position in a situation.
Looking for something in your locker can mean you are missing or seeking something in your life.Not being able to find your homework or a book can mean you may get into trouble in real life if you haven't already, or that you're not living up to your responsibilities (or are afraid you won't). Cleaning out or organizing your locker can mean your life could use some cleaning out and re-prioritizing. Not being able to open your locker, or not remembering the combination, can mean you're feeling stressed or afraid things won't go well, or that you are sabotaging yourself or holding yourself back from success somehow.