What does K-12 mean?
In the United States, students attend public school between the ages of 5 or 6 years of age (Kindergarten) and finish at about the age of 18 in high school grade 12. Grades K-3 (primary school), 4-5 or 6 (intermediate school), 6-8 or 9 (middle school or junior high) and 9 or 10-12 (high school). There are indeed some individual differences between states.