The phrase "school flame" refers to a person with whom you have a romantic or crush-like attraction who goes to the same school as you. Full definition
My husband also had an emotional affair with an old high school flame, with whom he met secretly for lunch dates for two years behind my back. (goodtherapy.org)
Or maybe I'd bump into her in a coffee shop near my house, or perhaps, in a fun plot twist, she would be an elementary school flame who resurfaced after college. (relevantmagazine.com)
After the funeral hank meets his high school flame Sam (Vera Farmiga) and makes out with fiesty barmaid Carla (Leighton Meester) while Joe goes for a drive. (independent.ie)