Toddy had it summed up in one sentence for me.
My longer version is my reflections on my life. Whilst I've so much to be grateful for, the greatest is my young son and partner, my regrets have defined me in many ways.
I was lazy but clever at school. GCSE exams at school was too easy. I didn't need to work so I didn't. It came easy and I got good grades if a little below average for a grammar school. A levels are harder so whilst I didn't struggle I still didn't work for them so failed to get the grades needed to study what I wanted to study for years. This leads to my first new bit of advice...
Don't be afraid to take a step back to find a new direction.
In my case I should have taken a year out and re-book my a levels. Doing that I'd be motivated to work for something. But I didn't because it felt like failing. I was the bright one with university expected from primary school.
So I rolled the dice and took the first course offer I got, a very difficult course but one that had no future. Mining in this country? We're talking metal mining not quarrying and coal was as good as dead.
That led to no career direction, lots of focus and ultimately into a rut. Then redundancy and a low paid job. Not bad for someone with two degrees right!
So my greatest advice to school kids would be to find your interest in a career then study towards that. Work when you're younger to make it easier when older. Get a good career and life in many ways becomes your oyster.
If you don't then you might be lucky or might not. It's better to make your luck through hard work early on in your life. If you've not made a good grade by 30 it's hard to make it by 40 IME.
PS of anyone's got words of advice for someone in their 40s more that is something I need!
TL: DR
See Toddy's post, first sentence.
Study, find a focus and pursue it.