Wiktionary
first grade
n. (context US education English) The first year of grade school, the period in school that comes after kindergarten and before second grade. Children usually begin first grade at age six.
WordNet
first grade
n. students beginning elementary school
Wikipedia
First grade
First grade (called Grade 1 in some countries) is the first grade in elementary school. It is the first school year after kindergarten. Children are usually 6–7 years old in this grade level.
Usage examples of "first grade".
By the end of first grade, a black child is underperforming a statistically equivalent white child.