The public school system was established in America so kids would grow up to be adults who could make wise choices as voters in a democracy.
When they begin attending school, children must learn to function as part of a group, to share, obey rules, and listen to those in authority. But they can learn those things in other situations.
By learning to read and write kids gain access to vast amounts of information that will be useful to them for the rest of their lives.
Arithmetic is essential for handling money and other tasks needed by all adults since calculators and other devices might not always be available.
As the saying goes, those who don't know history are doomed to repeat it, so Social Studies are certainly important.
Since many modern kids no longer just "go out and play" Physical Education is essential for their health.
Science helps them understand how the world works and provides practical information they'll certainly need as adults.
Art and music open their minds to appreciate the world around them and help kids discover and develop their talents and creativity.
Of course as they get older they'll learn more about all these subjects and get to explore others.
So, if kids schools could teach only one subject, which of these should it be? In my opinion, it would be reading because all the others, even how to participate in physical activities, can be learned through that skill.
What do you think is the most important thing kids need to learn in school?