This is the 7222nd most frequent Japanese word.
上級
"上級" means "advanced level" or "high rank" in English.
Here, "上級" is used as an adjective describing the class level.
彼女は上級クラスに入ることを希望しています。
She hopes to enter the advanced class.
Here, "上級" is used as a noun, indicating a level of proficiency.
彼は日本語の上級者です。
He is an advanced Japanese learner.
In this context, "上級" modifies "者" to form "上級者," referring to someone at an advanced level.