High school is not a proper noun. It is a common noun that describes a type of educational institution for students typically in grades 9 through 12. Proper nouns, on the other hand, refer to specific names of people, places, or organizations and are always capitalized.
For example, when you refer to ‘high school’ in general, it is lowercase. However, if you mention a specific school, like ‘Lincoln High School,’ that would be a proper noun because it names a particular place. In summary, ‘high school’ is a general term, while names like ‘Central High School’ or ‘River Valley High School’ are proper nouns due to their specificity.