If I have to choose a song, it would be "Let's get loud" by Jennifer Lopez. It reminds me of the end of the summer when school starts and when the leaves start to get yellow but it's still warm outside.
My favorite back to school track is and will always be ...Baby One More Time (1998) by Britney Spears. Not really because of the lyrics, but rather because of the great video we'll never forget. Back in the days I was preparing for high school when this track was released and the video was giving me some perfect vision of how the high school atmosphere would feel like.