high school

[Unregistered]

which one is correct? what is the difference? he is a teacher at many different high school not just one, so high school needs to be pluralized?

My father is a substitue teacher who teaches at high school
My father is a substitue teacher who teaches at high schools

[Casiopea]

at high schools: place
Meaning, he teaches at more than one high school

at high school: place
The meaning is awkward. Try,

Ex: My father is a substitute teacher who teaches high school.

Meaning, he teaches high school subjects at either one or more than one high school.

All the best.

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