Here's why:
* Compound nouns: "Seventh grade" is a compound noun, meaning it's made up of two words that function as a single unit.
* Hyphen use with compound nouns: Hyphens are typically used in compound nouns when the words are both adjectives describing a noun (e.g., "well-known author") or when the words are newly coined (e.g., "user-friendly").
* Established usage: "Seventh grade" is a well-established term and does not need a hyphen.
Example:
* Correct: The seventh grade students are going on a field trip.
* Incorrect: The seventh-grade students are going on a field trip.