Dreaming about the first day of high school as an adult can symbolize a variety of feelings and experiences. Here are a few interpretations to consider:

1. New Beginnings: This dream may reflect a desire for a fresh start or a new chapter in your life. It could indicate that you are embarking on a new journey, whether in your career, relationships, or personal growth.
2. Anxiety and Self-Doubt: High school is often associated with social pressures and self-discovery. Dreaming about it as an adult might suggest that you are experiencing anxiety or self-doubt in a current situation. You might feel unprepared or out of your comfort zone in some area of your life.
3. Nostalgia: This dream could also evoke feelings of nostalgia for your youth. It might be a reflection of your past experiences and how they shape your present identity.
4. Social Dynamics: Since high school is a social environment, this dream might relate to your current social interactions or feelings about belonging and acceptance in a group.

Consider what aspects of your life currently resonate with these themes, and reflect on any changes or challenges you may be facing. This can provide further insight into the meaning of your dream.