Dreams can often reflect our current thoughts, feelings, and experiences, especially during significant life stages such as adolescence. Your dream seems to encompass themes of identity, social dynamics, and perhaps a desire for acceptance or recognition in a new environment.

1. New High School: Starting in a new school can be a source of anxiety and excitement. Meeting the principal and lying about your grades may symbolize a fear of judgment or the pressure to perform well. It could also indicate a desire to make a good impression.
2. Pool Party and Social Groups: The pool party filled with "cool kids" suggests a longing for social acceptance and the desire to fit in with your peers. Water often represents emotions, so being in water with waves could signify feeling overwhelmed by social situations or changes happening in your life.
3. Transformation into Another Girl: Dreaming that you are in another person's body, especially one you perceive as attractive, might indicate a wish to explore different facets of your identity or to embody qualities you admire. It reflects a natural curiosity about self-image and how you are perceived by others.
4. Flirting and Attraction: The interactions with the girl and the guy suggest a subconscious exploration of romance and attraction, which is common during your age. It may be a way for you to navigate your feelings about being desirable or accepted in social circles.
5. Deep Sleep Feeling: Waking up feeling like you had a deep, transformative sleep could indicate that your mind was actively processing these themes. The heaviness you felt might reflect the weight of these emotions or thoughts.

Overall, your dream seems to embody your current experiences related to self-identity, social dynamics, and aspirations. Consider what elements in your waking life resonate with these themes and how they might inform your journey through this stage of life. If you have any specific feelings or questions about these experiences, reflecting on them could provide further insights.