It was necessary in order to have an educated populace. It was not designed to be a job training institution or to prepare students to be "college-ready." It was to help people become citizens of the United States.
Not that the "reformers" give a shit about it. Gutting money for public ed forces school districts to be even more reliant on billionaires' "generosity," which almost always has strings attached.
The privatization of education is profoundly antidemocratic and alien to the basic principles and conceptions that animated the American Revolution. Among the most far-sighted and principled of the Founding Fathers, the development of a system of universal public education was seen as a fundamental responsibility of the state and prerequisite for the survival of democracy. This view was rooted in the Enlightenment traditions of reason, science and popular sovereignty.
It was championed as an antidote to the aristocratic principle that prevailed in Europe and deemed essential to the egalitarian impulse spelled out in the Declaration of Independence’s revolutionary assertion that “all men are created equal.”
This is from a so-called Trotskyist site, but the commentary is far more conservative in the best sense of the word than the privatization bullshit we have heard almost non-stop for the past 30 years, ever since neoliberalism foisted its poisonous ideology on American politics.