I think I get it now. So at first, she's singing about how this is her school, and information about what's happening. Then she talks about being an adult. In the offical video, you see a woman in a white shirt (Different from the rest), and I believe that this would be her watching her past self on that day. The song keeps on going on and on repeating how she learned things when she became and adult. So it's basically a reminiscence of her past.
Indeed, it is a song about nostalgia. She is a school girl that wonders if she will be able to feel the same, laugh the same, talk the same and meet with her friends in the same way she does now when she becomes and adult. Quite relatable, the nostalgia
