The problem is that in the US we lump together multiple types of high school diplomas. In most school districts, you can specify either: 1) a College Prep track in which case you will take Algebra/Geometry/Trigonometry/Calculus, Biology/Chemistry/Physics, World and American History, English Literature, etc. or 2) a General Ed track where you will take arithmetic, science, social studies, basically 4 more years of middle school. Both sets of students get the same diploma. In other countries you have things like A Levels to differentiate the high school diploma holders who took hard classes vs those that took dum-dum classes. Without that, you end up with an information asymmetry like the market for used cars.