WHY DO SCHOOL CHILDREN MAKE UP 12/75THS OF THE POPULATION?
Imagine everyone lives to the age of 75. Clearly each person spends just as much time in one age year between 0 and 75 as in another (i.e., all years are the same length, and you can't skip any.)
Most people spend 12 years in school grades 1 - 12. Thus at any given time, roughly 12/75ths of the population, or 16%, is in school.
In reality, the birth (and immigration - emigration) rate is not precisely constant in time, and of course everyone doesn't have precisely 75 years and then drop dead. The census bureau's actual tally of the population by age can be found here.
49 million between ages 5 and 17 = 16.7% of the population. So 16% is not such a bad estimate.