I dreamed of a girl sitting down in her school desk. As time passed by, the girl looked at the mirror, realizing she's an adult now.
Dreams can often be interpreted in many ways, but here is a potential interpretation of the dream you have described:
The dream may represent a transition or milestone in the dreamer's life. The girl in the dream could symbolize the dreamer or a part of themselves that feels like they are still in a youthful stage of life, represented by sitting at a school desk. The passage of time and the realization of becoming an adult is a common theme in dreams. Seeing one's reflection in a mirror can represent self-reflection and introspection. The dream may be indicating that the dreamer is becoming more aware of their own maturation and the changes that come with it. It could also highlight the potential anxieties or uncertainties that come with growing up.