In Kindergarten Cop (1990), why are there kids above 6 learning inside a normal school? Are they special, or is something off? For example, the bad guy is searching a kindergarten for his kid, while stating his age is 6.
It's not uncommon for a 6 year old to be attending kindergarten, especially near the end of the school year. Most children turn 6 during the course of school year.
In most states a child must be the age of 5 by August 31st or September 1st to enroll in kindergarten. A typical school year ends in late May or early June, if a child entered kindergarten at the age of 5 and was born before May they would leave kindergarten at the age of 6.
There are schools that run Kindergarten through 8th grade, primarily in smaller towns where population is smaller and thus more grades occupy the same school to minimize educational funding.
Kindergarten Cop is said to take place in Astoria, Oregon, which (last census) had a population of less than 10,000. So, it's not unreasonable to see children from 6 to 14 walking the same halls.
The John Jacob Astor Elementary School was used for the exterior scenes of the fictional Astoria Elementary School in the movie.
On the school's current Facebook page it says:
Lowest grade taught: Kindergarten
Highest grade taught: 2nd Grade
But according to this article from 1991 (the movie came out in December 1990):
For nine days last June, many of the 400 students in kindergarten through fourth grade at the John Jacob Astor Elementary School got paid $35 a day to act as extras ...
So seeing kids older than 6 years at the school wouldn't have been unusual back then.