I can honestly say that the two most important things to learn and understand is school is: mathematics and english. If you can read and comprehend what you are reading; and can add, subtract, multiply, and divide; know basic geometry and algebra then you can also learn anything else.
When I got out of the Matines in the mid-seventies and went to Community College, the english professors were complaining that high school graduates did not know their grammer. What; I learned grammer in, oh, um, grammer school. Well I guess it was called elementary school.