I tried to address this in the limited space of the essay, but could only go so far. The core skill sets that we dedicate almost all our attention to in high school — math and language — are developed, practiced, and advanced in every class. Every class involves reading and writing, and every possible science class involves math. To kind of bypass this self-evident truth, high school treats the core subjects the way Universities do, as academic fields of professional study. But high school students are neither English or math professors, nor are many of them going to be!
Universal continuity is necessary if one is building a base set of skill and knowledge needed by every student. This is exactly what elementary schooling is about, and the system is structured to deliver it that way.
But when it comes to most of the classes that high school kids take, every year of progress separates students into a wider and wider range of interests and needs (not all of them, maybe even not the majority of them, specific to the demands of a college degree). We justify it by claiming — though with less and less authority or enthusiasm every year — that Calculus and Shakespeare should be mastered by everyone, when in fact we have little compelling argument that this is worth either what is required to make it so or the various consequences.