Dm taught in a small school where a fair number of the pupils were below average intelligence. She also did tutoring which varied from low ability through to high.
For her subject (maths), she said that everyone seemed to have a ceiling. However hard work and things like 1-2-1 tutoring did not necessarily increase the ceiling, but get them to that place. She had pupils predicted E at the start of year 10 who got a C. So it can be worth encouraging their work, and even getting a tutor to get those passes in English/maths if that is needed.
The hardest she found was either when the parents had unrealistic expectations ("they have to do higher level, as it's ridiculous that can't get higher than a C" when they'd be doing very well to get a C, and more likely to get a D-which would be just a simple fail on the higher level) or where there was a mismatch in the children of the family, because often tactless things were said with no malice: "no point doing something you'll only get a D in" was one, in front of younger sister who wasn't going to be getting Cs in anything.
What she did say was often the ones who weren't achieving brilliantly often had great ambitions which they realised, just not in the academic world. Of those who were struggling through school many of them have found jobs that they find fulfilling and they are actually great at. It's just a case of finding it. That's where you can support them.
I get my tyres changed at a local Kwik Fit. One time my car was done by a young school leaver. He struggled with the paperwork, and he was very nervous. I had ds with me who was about 2yo at the time.
I went back about 18 months later and he took one look at the car, stated "oh yes, you had X, Y, Z done, and how's your little boy?" His confidence had grown hugely and he not only remembered the car, what had been done, but he handled the paperwork with confidence and was totally a changed person.
How much better he would have felt if at 14yo he could have stopped most of schooling and been able to do an apprenticeship in car maintenance or similar. That's where I feel our educations system is wrong.
But going back to him, he moved on after about 5 years-he was promoted to manager at another garage. That's pretty good going for his age. He was planning that when he'd had enough experience there he was going to set up his own place. Good luck to him. I hope he's achieved it.