日本語教室 versus 日本語の教室


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 **の** 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!

