In 1941, in more or less a state of nature, 25% of American Army inductees had HS diplomas.
I take that to mean that 75% of American 18 year olds lack the native ability or inclination to complete 4 years of English, 3 years of History, Mathematics through Trig, 3 years of Science including Physics and Quantitative Analysis, and a Foreign Language.
Now, you can redefine high school competency to mean, "didn't get arrested, smiles at adults", in which case I agree the number could approach 100.
But if you think 100%, or even 50% of American 18 year olds can approach the level of mastery to get a 1941 HS diploma, I think you are dreaming.
The result of the "100% can succeed" mentality is a system based on lies, which have to get bigger and bigger every year to cover up the last generation of lies.
It's time to stop, and make high school optional for anyone with an IQ less than 95.
If you're going to allow for some type of training beyond eighth grade, I would agree. Some people aren't crazy about abstract concepts (my dad was one of them), but can build a house from the ground up, wire it, plumb it, and live happily ever after in it.
I'm lousy at building things. Even simple Sauder Furniture bookcases give me the fits. But the Vo-Tech guys I went to school with -- the guys who thought algebra was a type of fancy lettuce -- are master mechanics, furniture craftsmen, electricians, computer hardware specialists, the list goes on.
Everyone needs to learn to read, write, and make change. Beyond that, everyone needs to be directed to training that best suits his or her temperment. Whether the student and his parents choose the 9 - 12 grade curriculum, or the school tests and places the student is another matter for debate. But I do not think high school is optional, not when courses can be designed to fit the student and train him to be a productive 18 year old.