Listening to songs from that time in my life are perfect for getting the emotional memories flowing. Often from the very first note, my mind will go back to an exact place, a situation, maybe even a heartbreak or injustice--or happiness experienced by my twelve-year-old self. Feeling and emotion are everything in story, and airing them out again can spark something worth writing.
If you live near your old stomping grounds, try taking a walking tour with your notebook. Or if you are far away, try a virtual walk using Google maps. You may be surprised at what you see around your old neighborhood. I once looked up my old house on Zillow. Just the sight of my front yard where I used to sit in the shade of a giant fir tree for hours with my stack of library books brought up so many hidden gems of emotions.
When was the last time you dusted off your old middle school yearbooks?
There can be a wealth of emotions and angst, hopes and fears, scrawled in the purple and pink comments of the endpapers. (Caution: Don't get caught up in the time suck of tallying up the number of times it says, "Have fun this summer." ... or .... "I hope I get you in some classes next year." There's also the classic, "Don't ever change.")
Give it a try. Go back to the pages and sounds and streets of your middle-grade self. You may end up with the perfect detail you'd been searching for. Maybe you'll end up with an entire story . . .