From the Current Population Survey:
Is (Person) attending or enrolled in regular school?
What grade or year is (Person) attending?
Was (Person) attending or enrolled in a regular school or college in October, 201_, that is, October of last year?
What grade or year was (Person) attending last year?
What is the highest level of school (Person) has completed or the highest degree...has received?
High school completion is the percent of persons aged 18 to 24 years not currently enrolled in high school who report that they have received a high school diploma or the equivalent (such as a General Education Development (GED) certificate), regardless of the type of credential.
States are able to report in 3-year averages only. For States with small populations, the 3-year estimates may be unreliable.