Here's why:
* Compound nouns are formed by combining two or more words, where the resulting word acts as a single unit with a new meaning (e.g., "bedroom", "firefly").
* Hyphenated nouns are two or more words joined by a hyphen to create a single concept (e.g., "high-speed", "well-being").
In "high school", the two words "high" and "school" retain their individual meanings but combine to describe a specific type of educational institution. The hyphen serves to link the words and emphasize their combined meaning.