There is no such thing as a 12th grade level. 6th grade is all the farther English is taught. There are sometimes things called levels above that, but they are not levels in English they are levels in some specific specialization and have more to do with that specialization than reading. (doctors would not be expected to understand deep computer science writing even though doctors have very advanced reading skills in a medical domain).

I understand Spanish only goes up to a 5th grade level while Japanese to 9th. This is a reflection on how complex the writing systems are, and not the kids, intelligence, or school systems.

The years after you have your basic reading down are used to learn other skills that are of general use in life for everyone.