@abge,
I agree that the right way to solidify knowledge is to build on it in later courses. I do *not* agree that taking any course a second time would be ideal. It would be enormously boring if you had learned it well the first time.
Let us turn to the two advantages you present for taking calculus in high school:
"1) It gives you a taste of more advanced math, which is helpful for determining your career."
That seems reasonable, for those undecided on a career. It doesn't help your original claim at all, though, which was that, "by the end of HS they better be able to [integrate] if they want to be an engineer."
If we're talking about somebody who already wants to be an engineer, then this point simply doesn't apply to them.
"2) It gives you an opportunity to see the subject more than once, which I still maintain is helpful, even if it isn't always practical."
OK, I'll agree that for some it might be helpful. What I don't see is how this "helpful"ness reaches the level of "if you want to be an engineer, you'd better do it." If you understand math well, you're going to be able to do calculus, whether it's at the college level or at high school. It might be nice to have seen it before, but it just doesn't really matter.
Let me try to explain some more my perspective on this stuff. Over the years, as I've tutored, taught, and graded the exams of many hundreds of calculus students, I have noticed several different types that I see a lot*:
1) The ones who don't really understand algebra or trig, and so simply cannot complete a problem correctly because they haven't internalized the rules or what they mean, and they cannot manipulate expressions. This is quite a large portion of students (including many who have taken calculus).
2) The students who are solid on algebra and pretty solid on trig, but who have conceptual problems with calculus, especially word problems and reasoning about derivatives, integrals, etc. in the real world. This is a sizeable portion.
3) Those who are excellent at math, have internalized what is going on, and quickly are able to use and apply calculus. This is a small minority.
*I'm creating this broad, rough classification by thinking back now. I *of course* do not classify students when I am interacting with them, and there are many personal variations, differences, etc.
Now, the students in group 2 can get there. I am not ultimately concerned about their performance, if they work hard. They are the students who seek help and are able to be helped.
The students in group one, on the other hand, face horrible odds. They can occasionally work so hard that they catch up and make it through well enough that they have some chance at continuing in science or engineering; but the odds are terrible. I have graded hundreds of their exams, and it's a depressing and awful experience. They are arriving at the last days of their hopes of being engineers, and there is little we can do for them. (Of course there are outliers. I know at least one person who struggled mightily with algebra early, had to go to junior college to take developmental math, and ended up with a mathematics major. Gloriously, it does happen, and it should always be our goal).
These experiences greatly impact my view of what is *crucial* for somebody who wants to be an engineer or scientist. It is crucial that you come to college calculus knowing how to do algebra and trig. (Take precalculus if you can't do trig). If you know that, then we really can teach you calculus. Maybe it would be easier if you had seen it before, but you can do it, and it's not so likely to develop into the roadblock to your career. The better you are at algebra, the more unlikely it is. It's very rare to see student wipe out on calculus without bunches of algebra errors, if I've ever seen it at all.
So that drives my perspective on the necessity of taking calculus before. I can't even visualize a student who fails calculus because he hasn't seen *calculus* before. I live every day among students who fail it because they don't really get algebra.
"So, agreeing that a 4 year degree is already impractical to obtain in under 5 years, how can you say there's 'There's plenty of time to take it in college'? "
Yeah, because I don't really think that's mostly because of calculus. Of course there are hundreds of universities and tens of thousands of different students situations, so doubtless there are some for whom taking calculus in high school is the difference of a year in college. That's certainly something to take into account, then; but given your remarks, and the discussion I've given, I take it this can't be the primary motivation for a *necessity* of taking calculus in high school.