GIGO

See conclusion #3, here. Be sure to follow the link to the 8th-grade exam from 1895.

Clarification: Conclusion #3 (which is part of a post at Carpe Diem) and this post are about public education. "GIGO" stands for "garbage in, garbage out" -- a phrase that was common in my early days as a defense analyst. It's a shorthand way of saying that the results produced by a model will be erroneous ("garbage") if the model itself and/or the input values chosen to represent the model's parameters are ill-founded or empirically incorrect ("garbage"). (Much like today's climate models.)

I don't mean to refer to today's public-school students as "garbage" (though some undoubtedly are just that). What I mean is that they are taught too much "garbage" (socially relevant clap-trap, sex education, etc.) and, therefore, not taught enough readin' (including Latin and other languages), writin', 'rithmetic, geography, and history* by teachers who actually know those subjects. That has happened largely because public education in this country has been taken over by a cabal of university "education" departments and teachers' unions (both Left-wing), which dictate the kinds of clap-trap being taught in (most) schools and discourage the thorough training of teachers in those subjects that are worth teaching (readin', etc.).

As for the students, their main deficit -- aside from having been let down by the "system" -- is the growing absence of one or both parents, because the of the marked increase in the incidence of working mothers, divorce, and illegitimacy over the last 50 to 100 years. Today's students (on the whole) therefore suffer (relative to their predecessors of 50 to 100 years ago) from a lack of parental interest, guidance, and compulsion.

I haven't discussed manners, obedience, and violence because the differences between now and 50-100 years ago are too painfully obvious.

Related reading, here.

More about public schools from Carpe Diem, here. And more.
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* By history, I mean not only "history" but also something modeled on Kenneth Clark's Civilisation, with the addition of instruction in the formal development of music through the nineteenth century (the rest is noise). Desirable options: instruction in the performance of music and creation of art, as long as its music (not noise) and art (not doodles and blobs).