"K-6" is a common abbreviation for the grades in school from kindergarten to sixth grade, which is typically the first seven years of a child's formal education. The concept of K-6 is notable because many schools only group kindergarten through fifth grade together. This system is referred to as "K-5."
There are many arguments as to why sixth grade should be included in elementary school, as K-6, instead of middle school, or K-5. A 2007 study showed that sixth graders in middle school have more behavioral problems than sixth graders in elementary school. Other studies show that sixth graders in the K-6 model test higher than those in the K-5 model and that K-5 sixth graders are exposed to older teens who can have a negative influence on them.