I'm always telling teen-agers that is great to work towards their dreams but what's the back up plan?
So Billy wants to be a footballer. Fabulous. Work hard on your football, make sure you stay fit, etc. What's the backup plan for if you get injured and can't play? Make sure you're also working hard on your school work so you've got choices about that backup plan if it's needed.
Betty wants to be a vet. She's currently in set 4 for maths and English, and is unlikely to get any higher than grade 4s in her GCSEs. Ok Betty, work really hard, but it's very, very competitive to get into uni to be a vet. What else might you like to do that involves working with animals? What about vet nurse, or working in an animal refuge, or getting a job on a farm, etc?
Both of those children are fictional, but based on any number of kids I've taught. I agree with pp that the most difficult children to work with are the ones with no aspirations. They are the ones who are least likely to work hard at school and get the grades that give them choices about their future.
Obviously getting good grades at school doesn't automatically get you everything you want. But it does put you in the position that you are more likely to have choices about what you want/can do. A lot of jobs are blocked if you don't have grades 4 in English and maths. Quite a lot of jobs require some sort of degree. Some jobs require very specific degrees with post graduate study on top. So the better you can do at school, the more options you have in terms of future study and/or employment. That doesn't stop live circumstances happening to mess things up for you, though, that's bad luck, and can happen to anyone. Illness, traumatic experiences, bereavements, etc can mess up even the most well laid plans unfortunately.