I feel I have to add a bit about my own experience, for some balance (although OP may have already left!).
I went to school in a system that doesn't have one or two all-important set(s) of exams - whether or not you get in to university is based on an average of the marks you earned in high school, which are themselves a (weighted) average of the coursework and exams you do throughout a course. It's more of a long slog than a short but intense period of exam stress.
However, I don't think I started a single piece of homework before the day before the deadline until I was into my 20s. I didn't study for tests or exams for more than an hour or so until university. At the risk of sounding vain, I have a fair amount of natural academic ability and the absolutely minimal amount of work I put in was enough for me to (usually) get very good grades. I know this approach wasn't popular with my teachers, who often told my parents I would "hit the wall" once I got to university. My parents were usually pretty hands-off in my education and my general upbringing (the prevailing attitude was "it's your life, and you're the one that has to deal with the consequences"), but they were supportive when I needed or asked for it. But I absolutely hated it when they tried to help with my homework. It would inevitably descend into shouting matches, hours-long lectures, tears, etc. It was so stressful, much more stressful than doing it the way I normally would (rushed and at the last minute). Eventually they got the message that I would be fine on my own, and relations improved significantly.
I probably could have done better if I had let them do more for me academically, but our relationship was just not in the right place for that to happen. Additionally, at least my way I felt that my achievements were my own - if I had only done well because mummy and daddy helped me, then how could I really say I had achieved those things.
Anyways, in my late 20s I now have three degrees including a PhD, so things worked out in the end. I even (sometimes) start things before the absolute last minute - something I was able to learn myself, through experience.
I also want to say that I think it's possible to overstate the importance of GCSEs and, especially, A levels. If a kid doesn't do well in exams, then things are likely to suck for them for a while (dead-end job, low income, low self esteem, etc.). But there are always ways back into education if they want to get there. The OU is a fantastic resource, and many universities will disregard A level results after a period of time, in favour of looking at what an applicant has done since high school or college (work, self-study, etc.). Getting bad results, or even getting no results, is not the end of the world if a person can still develop the motivation and desire to get back in to education. Indeed, lots of people are better suited to and get more out of university study in their 20s or later than they would have at 18.