In my opinion, the biggest problem in U.S. mathematics education consists of our textbook-publishing cartel and the bloated junk books that are being written, published, promoted and adopted. In many cases these junk series of books are adopted based on the recommendations of assorted highly-paid "consultants"--hacks who are hired by the incompetent members of Boards of education.
If we had sound textbooks, even a mediocre teacher would do a reasonable job at teaching the material, and the better students would be able to read the books themselves.
With respect to teachers, we must keep in mind that many of them must follow the directives of "curriculum specialists" and "directors of instruction.". These are petty careerists who usually hold the position for two or three years before they move up in the administrative ladder. Teachers must constantly switch to the shifting fads of these csreerists.