> For the people who don't need to realize that a 20% discount on $37.50 is $30, why bother sending them to school past 6th grade at all?
Now that I see your argument more fully, it seems less like an argument against testing and more like an argument against wasting resources on unneeded education. Do you believe the people who don't need the specific topics you mention actually do need education beyond grade 6? If so, why?

It's an argument against a specific form of testing. Frequent standardised tests; testing the same things across all the schools and all the students; using that as your measuring stick, shapes the fact that you're going to get a lot of unneeded, even harmful, education. And the more frequently you run those tests, the better your 'data' is (data which is not politically neutral, as some might think) the more pronounced the effect. Test scores are the incentive.

There's two answers I can give to your question. One is the practical answer, as things stand at the moment. The other is the idealistic answer, as things could stand.

For the practical answer, the most that I can say about someone who matches that pattern persisting in education, in the current system, after grade 6 is that they may be getting something else out of some of the other subjects that they study there. If you were a parent, and you couldn't afford to educate your child yourself, then it would probably still make sense to send your child to school beyond grade 6. If you don't do so, they won't have the qualifications to get by particularly well in life, and they may learn something useful in the process (though I wouldn't bet too much on it.)

Besides, you still need some where to stick your children while you're at work. If nothing else, education is free child-care for poor people.

That's not a good argument, nor is it an argument for the current system over the potential alternatives, mind. It's just saying that sometimes crap's the best thing on the menu.

For the idealistic answer: What I'm advocating for is a different model of testing, in which the statement that someone would need an education 'beyond' grade 6 is not as meaningful. There are different skills, different areas of expertise that they could go into, but it would be very difficult to rank those skills: Is someone who we've certified can refit a central heating system more or less skilled than someone who we've certified can rewire a house? Someone with both is more skilled than someone who only has one, but at the same time are they a better electrician than someone who only has one? Is someone who understands the mathematics underlying musical theory more or less skilled than someone who understands the mathematics underlying computational complexity? If you can apply Bayes Theorem, are you more or less skilled than a frequentist statistician? It's a lot harder to rank people when you throw out the idea of 'this year,' or grades, and just have specific skills and people who can or can't do the job.

Ideally I'd be able to answer your question by saying that children are given the chance to immerse themselves in a wide range of different subjects and projects - to follow those that appeal to them as far as they can go so that when they leave they have a wealth of ability that carries them through their lives. That if you're interested in mechanics, for example, you should be able to study mechanics all the time you're at school. Wouldn't we have some fantastic mechanics if they'd been learning on cars and lorries and boats and so on every week day for five or ten years under coaching appropriate to their skill level? Wouldn't we have some fantastic builders if they'd been learning building under that setup? Fantastic artists, fantastic musicians. Especially when we could focus on those who were interested. And who knows, they might - as they get further in that area - discover that maths is useful, and then they could go and get some maths badges when they've got a passion and a reason to drive their learning, (rather than it being something forced down their throats with no real promise of a relationship to their actual interests materialising.)

But I can't - and so the former answer is what we're saddled with. Do you need it? Sure. Unless you've got reasonably well-off parents, you've got to go through the paces. Should you need it, in that form? No.