Wiktionary
sixth grade
n. (context US education English) The period in school that comes after fifth grade and before seventh grade.
Wikipedia
Sixth grade
Sixth grade (called Grade 6 in some regions) is a year of education in many nations it is the last year of primary level (elementary). It is for students ages 11–12.
Usage examples of "sixth grade".
But I have to admit, when I hear about guns and drugs in the sixth grade, Bartlet starts to sound better and better.
She had been left back so many times that at fourteen she was still in the sixth grade.