Hyphens, En Dashes, Em Dashes
Q. What is the correct way to hyphenate school grade levels, “fifth-grade” or “fifth grade”? And “fifth-grader” or “fifth grader”? Thank you.
A. Nouns like fifth grade and fifth grader are both left open:
Not all of us made it past the eighth grade.
Some second graders are more advanced than others.
Adjective forms, however, are hyphenated:
Our fifth-grade lessons included more writing than reading.
See also CMOS 7.89, section 1, under “number, ordinal, + noun.”