Like everyone else here has suggested, I think that it would be important for parents to discuss this thoroughly with their kids, if they really didn't want to continue on in school. Whatever young adults choose to do in their lives, I think it's crucial that they know they've always got their parents' love and unconditional support and mentoring.
Kids in Australia today are SO fortunate, also, since there are so many educational and life options available to them, and also that there is a growing understanding that education can be a lifelong journey...not just something that you cram, half-heartedly, into your life before you're 25. For example, just because a young person drops out of high school doesn't mean that they'll never go back to study. They might just want some time out for a year or two to reflect on their life and where they're going. I've met many young people who've done this, and they all claim that the break gave them added clarity, self-knowledge, maturity and determination.
Although academic education is so intellectually enriching and opens the doors to a huge range of fascinating career paths it's not the 'be all and end all' of life...as some narrow-minded educators and parents present it to be. Like Jennifer said, some kids simply aren't academically inclined and would be much happier dropping out of school and learning a trade. With others, maybe they need to travel and learn more about the world we live in. In my opinion, world travel is often the best education, as we leave behind our comfort zones and all that is familiar in order to learn more about ourselves and others.
Back to the question, though. As others have pointed out, we're all individuals, with our own desires, dreams and personalities, and parents need to be aware of this in order to maturely mentor their young adult offspring to be the best, most loving people they can be. Our own dreams and expectations should not come into this. Always remember that success in life doesn't necessarily translate into academic 'success'. As parents we need to acknowledge our children are unique individuals who are on their own life journey, We cannot plan and orchestrate their lives for them. There are so many people in the big wide world who hate their lives and careers, and unfortunately even some high-flyers who have dutifully fulfilled their own parents' dreams and expectations for them, fall into this category. Let's not condemn our children to a life like that.