Being back at school in a dream is a classic sign of something being undone. When you’re back in school, you’re typically there to work. The mind sending you back to school can be because you’re slacking off, and you need to work on something that you’ve been neglecting. Or because your mind wants you to reconnect with some childhood vendetta that was left blank. Perhaps you’re missing something you had in your childhood? such as your parents, or a friend? The ability to be free with no worries about life? Or just overall the enjoyment of being a kid, and you miss the concept of childlike freedom.
In any case, this dream most likely is representing your mindset as a child. It has some relation to how you behave and the way you act during your childhood. It may be something you miss, or something you’re longing for.