It seems that the level (or quality) of education has been declining not only in the US, but throughout Europe too. The main reasons are more or less similar here and there.
1. Declining professional level of teachers. A teacher is not a desirable career anymore (at least in public schools). More intellectually and professionally capable people choose colleges, private schools, private educational centers, or completely give up the carrer for better paying jobs or better opportunities.
2. Education per se is not considered anymore as a step towards better-paid jobs and a social position.
3. A teacher is no longer viewed as some kind of moral authority for children and their parents. Various reasons for that - both paragraphs mentioned above; except of that, a 'freer' society where older and more experienced people aren't 'authorities' for younger generation, etc.