Even 100 years ago an 8th grade education was comparable to a 12th grade education today. My great grandfather graduated from 8th grade and he had read all the classics, could do advanced math, could speak and write English better than most college graduates today.
There is no reason to prolong childhood as long as we do today. At age 16 you should either be in college or in a trade school or apprenticeship in a trade.
You are right. High school is largely babysitting plus teaching things that should already have been learned by Grade 8.
For an example of what you wrote, read the 5th or 6th-grade McGuffey’s Eclectic Reader. The vocabulary and content would be considered at least high-school level by today’s standards and most high schoolers would have trouble reading them let alone answer the questions following each lesson.