When I first read your post, I thought of the Jay Leno show. He often does "Jaywalking" Jay will go out on the street and ask simple questions such as what ocean does California border. Don't be surprises if the answer give is the Atlantic. In one sense this segment is very funny, until you realize that these folks do not know simple facts. (ask them about pop culture...) The problem is worse when a person states they are a college history major.
I would say there are many reasons. Some will say that schools are not doing their job. The State requires many activities that are not academic. That could be part of the problem, however it goes much deeper than that. Many children are "latchkey kids". Another words, both of his parents work, and there is not one home to help the student. When a child comes home alone do you think they will do homework or watch cable TV that has 500 channels?
Our country is obsessed in high school sports. Some athlete-students will play 3 sports per school year. I wonder where they have time to study. Here in New York, they now have State playoffs. That requires an additional three games, and often the teams must travel several hours for these playoff games.
The Federal govt has required the "No Child Left Behind Act". Many times school teachers has to spend extra time with under-achievers, that the overachievers are not able to excel the way they could. Our society is getting to the point that we do not want to make children feel bad about themselves if they do not measure up. In fact, some schools do not allow a teacher to use a red pen, which traditionally meant a failing mark.
Discipline, or should I say a lack of; is a major problem. It is against the law for a school to give corporal punishment (spanking). And many states consider it child abuse. Its getting to the point that if your look at your child the wrong way...
I can tell you one thing that is not the problem - MONEY! Many will say that the schools need more money. I don't think so. Look at many Catholic and Christian schools. They are able to give their kids a better education, at less cost. But then they do not have all the expensive programs that public schools have.
There is more I could write but this is a start.
Salty