and there is no "6th grade level" that can be defined, or a least there is not one that everyone would agree on.
If you actually teach in today's schools, you'll see all kinds of kids speaking all kinds of languages with a wide variety of skills. Even though US public schools have always had the goal of creating a literate and numerate population, that has sometimes included teaching classics, latin, and logic. Handwriting is rapidly going away in today's world, while information searching is becoming a critical skill. There are a wide variety of sciences, languages, and fine arts in some schools. Home economics and shop used to be required, along with PE and geography. Many subjects that are required change over the years, along with the exact content taught under the same title. My mother's algebra class from the 1940's would look very different from today's technology driven version. I even used a slide rule in college science courses!!
A HS diploma is functionally a "certificate of participation" that shows completion of a certain number of years of school and finishing a given number of required classes. It is NOT an assurance of any particular skill level. Hopefully, the student learned enough to have a basis to continue learning either in the workplace or in higher education. About half the battle is simply learning to be a good student.
If you wanted to turn schools into a guaranteed skill level, you'd have to have absolute agreement on exactly what skills and knowledge everyone should be required to learn. That is virtually impossible.
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