The usual objection to this is people's aversion to having children at vastly different levels of physical development in the same classroom. Personally, I don't know whether to buy that argument, but it sounds worth expanding upon.
> Students graduate school when they have reached a certain level of competency in all subjects. This could take an arbitrary amount of time. e.g. it might take one student 3 years but another 8. But once a student graduates they will be competent.
OK, but at age eighteen, physically preventing them from leaving is against the law, and will likely be punished.
I'm only half-joking:
At some point, the law recognizes people as full adults, and it is much more difficult to legally restrain an adult, even if they're reading at a fifth-grade level.
Personally, I think the answer to that is more and better trade schools, and increased governmental public works programs, but such ideas went from being All-American in the 1930s to Socialism in the 1950s, and we're still not quite past that yet.
I think the future is more automated, independent learning. At my kid's school, a good chunk of the math curriculum is taught by computer. They go to a lab, and get to play math games that change according to their success with what they've been presented already. So if a kid has trouble adding small numbers, he'll get more of those problems than a kid that has that figured out, and can move forward to adding large numbers, multiplication, division, or whatever else still challenges him, regardless of his age.
This also handles attention span issues. If a kid can be challenged with new exercises and learn for an hour and a half straight, great. If another one has trouble learning new material late in a class, he will do worse, and face easier material as the class goes along.
Socialism was "All-American" in the 1930s. Then history happened and it garnered itself a rather nasty reputation, which it went to great, great effort to earn fair and square.
Re your second point. It's just a graduation, no one is forced to stay in school.
how about physical education (i.e., sports)? you'd usually want to group similarly aged students in those, lest you have a 16 yr old competing against a 10 yr old.
Like the CNC machining meme. If you reengineer society so that everyone below the top 5% of their high school class becomes a CNC machinist... it's not going to be a rare and valuable skill anymore.
On the surface it seems like you're arguing that there is not enough room in the trades for more tradesman. However adding a good chunk of people to the trades will increase the velocity of money in the form of more people spending more dollars based on their higher salaries. The alternative is that the demand for trades is fixed, which doesn't make much sense to me.
The demand for trades may not be fixed, but it is not proportional to the number of tradesmen. Most likely it's proportional to overall economic activity in some way. Doubling the number of tradesmen will not double economic activity across the whole economy, or likely have any significant impact on demand for tradesmen, so the value of tradesmen will drop until some are forced out and an equilibrium is reached, probably not far off where it is now.
You can have a minimum of 50% and still have kinds know 10 times more than what a minimum of 100% course has them.
50% of an advanced course is better than 100% of a course for idiots.
It's all about the breadth and deepness of the material, not some arbitrary mark on it, like 50%.