Alas, the trend is moving in the opposite direction—they are raising the age to legally “drop out”. The real reason has little to do with education—it is to keep ferals “off the streets” for as long as possible, ie. baby sitting.
I taught junior high for over thirty years. Two hundred years ago, my 7th and 8th graders would have been considered adults. The girls would be getting married and the boys would be apprenticed out to learn a trade. We, as a society, for better or worse, have artificially extended “chlldhood”. It is one reason why those years are so difficult. These “kids” are or can be so much more capable than we think.
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.