The purpose of mass education

Mass education has two components. The first is to impart knowledge to
children that they need to effectively function as a member of society.
This includes literacy, basic arithmetic and a basic understanding of
civics in order to participate in our republic. At a higher level this
imparts technical knowledge to various professionals, technicians and
artisans.

The second function is a sifting process, designed to separate the wheat
from the chaff. It recognizes that not everyone can become a doctor,
lawyer or engineer. It gives employers a basis for choosing the most
appropriate employees for various jobs.

Generally speaking, no matter how much you educate the population, human intelligence and skills are distributed along a bell curve. The talented 10 will tend to land the best jobs while the least talented at the other end of the curve will get the least desirable jobs.