"kou hoa papa" looks singular. Why are both acceptable, "classmate" and "classmates?"
Is the plural kou mau hoa papa*?
Yep, kou mau hoa papa. Classmate (singular) would have been better here unless the context indicated otherwise. Could also say E nīnau i nā hoa papa. In class, we usually say E nīnau i kekahi hoa papa.
The previous prompt was in English and indicated that hoa papa could be singular or plural.