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I know that it's correct to say 日本語教室, and it's also correct to say 日本語の教室. I am confused by 日本語教室. From my understanding, the particle の connects 2 nouns. For example, さくら大学学生 (there is no particle before university because さくら大学 is one name).

Please explain why 日本語教室 is OK. When can I omit の? And when is it impossible to do?

Thank you very much!