Okay, so a measure of time is not some hairy green monster calculated to frighten children. But the Carnegie unit should frighten the American public, if the latter realized how it, though well intended and once possibly useful, is a prime player in condemning our secondary schools to second-class productivity.
Once upon a time, secondary schooling was rare; fewer than ten percent of an American cohort went to high school in 1900. But between then and 1950, high schools popped up all over the country, and attending and completing education through them became more and more normal. The Carnegie Foundation wanted to encourage high school graduation, as well as college attendance for promising pupils, and spent millions pursuing these ends; but newly expanding colleges found themselves troubled when trying to evaluate applications from students whose schools and districts used every variety of transcript summarizing the most heterogeneous high school programs. The Carnegie Foundation, trying to impose order, establish standards, and assist comparability amid this chaos, decided that a good strategy would be to mandate a set minimum of time for high school students to be in class with a teacher in order to receive credit for their labors, and they bought assent to their proposal by funding retirement benefits to teachers who would back the standard. Thus the American system of "credits" for seat time, unknown in other parts of the world, was born; the Carnegie unit was the foundation of our system of credits. (An interesting parallel is that the European Union, confronting diversity in its higher education systems similar to what Americans had in secondary schools a hundred years earlier, has recently resorted to the same solution through its European Credit Transfer System.)
But how is this monstrous, you might ask? It forces our schools to offer a few subjects (English, mathematics, science, social studies, and to a lesser extent foreign languages) in roughly equal proportions in all four years of high school, in a dull, monotonous routine: "Period one I have algebra, period two science, period three English", and so on, an American student might say, regardless of the day of the week. By contrast, if you asked European or Asian students for their schedules, they would have to ask which day you were referring to, since it would change depending upon the day of the week; and particularly favored subjects, or those particularly necessary for a student's future, get more time than others, which appear in the schedule largely to ensure breadth, and at least a minimum attainment of some essentials in subjects which are likely undeserving of all the time necessary to earn a Carnegie unit (typically around 50 minutes five times a week for a school year): information technology is a good example of the latter.
Another problem is that our system grants credit for class attendance and teacher satisfaction (reflected in what we call "grades"), rather than for verifiable, public demonstrations of learning, such as external examinations.
Therefore a European or Asian student may well be studying ten or eleven subjects while our best are studying six; and their future mathematicians may be studying as many as 400 hours of mathematics, or fewer than 100, depending upon student desire and aptitude, in a year when our most and least enthusiastic, and everyone in between, will be studying (except at One World Secondary School) around 150, in all five or six subjects, unvaryingly, thanks to an innovation a bit over a hundred years old and a long way out of date.