What if grade level varied from school to school, and from culture to culture?
What if 40 years ago, five year olds played in school all day and had a nap, and now five year olds are supposed to sit at desks and do "school work"?
What if children used to physically play and be outdoors for a lot more hours than they are now?
What if the standard accepted things that students are "supposed" to know at each grade level are not necessarily logical or a natural progression? What if standards kept changing every few years or every generation? What if they tried teaching math one way, and then the next generation they switched, and then the generation after, they switched back? And what if different countries do a whole different method and timing?
What if the things that "most" children are able to do by xyz age are actually things that a growing number of children need to be medicated in order to do?
What if kids from some schools came into high school being really strong in math, and some kids came into high school being really strong in science, and all the different kids come in from different schools with different strengths and weaknesses based on the school they went to?
What if children who move to a new neighborhood and switch schools find that they are a little off, ahead in some areas and behind in others, in their new school?
What if a motivated post-high school student can start from scratch and learn to read and understand Hebrew and Aramaic in two to four years? What if a motivated student can learn the entire 1-12 mathematics curriculum at college age in one semester--less than four months?
So what are you so worried about? Relax.