Yeah, agreeing with editor. The joke falls flat and makes the story meaningless. If you understandably doesn't want to go for the "nerd gets hottie" trope, it needs to flow more naturally and still provides some kind of fulfilling conclusion. They become friends, rivals, perhaps the nerd finds his calling as a teacher, feels proud as a tutor or the delinquant starts tutoring other difficult kids as well. Any kind of pay off, really.
With that said, I wish the author showed us the end result. Guess himself couldn't envision a fulfilling conclusion when delinquant admits she bottled her dislike of him just for the sake of her grades.