They cover a broad range of topics in the most minimal manner possible. That, and every one of the people are dunces.
For example, in the 4th grade, I was taught what an atom was. "Building block of matter", and "Matter = Stuff". From 6-8th grade, (assuming they don't return to a previous unit in the last month of school...) we "learned" that an atom has protons, neutrons, and electrons. The protons and neutrons are big, and the electrons are little and go in a circle.
I, for one, could've told you all of that in the 4th grade. Even if I didn't, to mention that would take an extra ten minutes and would hardly be "over my head". Instead, they repeatedly teach the basics with a little bit added on each time. Excruciatingly little.