Speaking to The AP, the 23-year-old 'Price Tag' singer explained that she and Adele shared an artistic bond that drew them to one another.
"She was in music and I was in musical theater," she said. "We used to jam at lunch time and someone would play guitar and we both would just sing… At the time we both didn't know we'd have, like, albums out at the same time."
Jessie J went on to profess that she takes great pride in her former schoolmate's success, adding that Adele has always been an affable sort.
"She's killing it," she said. "I'm so proud of her... [At school] she was very kind of loud and everyone knew her and she was the girl everyone loved and up for a laugh, and you could hear her laugh from a mile down the corridor."
Last month, Jessie J claimed that her early struggles helped her to become the artist she is today.
Watch the music video for Jessie J's 'Price Tag' below: