Not sure if my experiences will resonate with anyone but I hope they can help. I've got two teenagers, boys, and the oldest is now doing A levels. Had the same issue of being bright as a button (especially with smart-alec comments!) but made minimal effort when it came to school work.
Eventually we sat down over dinner and had a 'grown up' chat about school, future plans, goals and ambitions. It was a slog, not many 15 year olds are willing to express any shadow of caring about their own futures, plenty of "I dunno's" but after a while he responded to being treated like an adult and we established he really enjoys languages and travelling and that he would love to work abroad.
I found that gave me a bit of ammunition to make him think of school work, exams and qualifications as a "means to an end". I made him research language based careers, language degrees, everything from translation and the glamorous life of interpreting (and how much they earn!) which seemed to excite him a little more than the idea of "growing up an getting an office job."
Tricky part was the actual catching up with school work. He was studying French and German at GCSE, is French was fine but his German was probably below par as he didn't get on with the teacher and other such excuses.
Time was running out and he desperately needed B's to study them both at A level, so I sent him on a German revision course that was being run at a local school (Justin Craig Education- think they do courses in every subject, and for A level as well) I wasn't really holding by breath but it seemed to help him a lot as they covered all the basics, revision techniques and exam technique, which is probably where most 15 year olds fall down even if they know the material - I can't imagine they've never had to cope with that kind of stress before. Plus he made a few other German-enthusiast friends (hoorah!) which seemed to make him think it slightly more cool to like languages. Plus as they were examiners and not his usual school teachers, I think he actually paid attention.
My advice never underestimate a teenager's real feeling as opposed to school-boy bravado - and try and make them see the bigger picture. Exams after all are to help them achieve their own ambitions. Also it turned out my son had been aware he was getting behind, but too stubborn to ask for help and decided to pretend he didn't care instead. Very glad I persevered - he got the grades in the end.