Is good student a idiomatic expression?

"Good student" is not an idiomatic expression. It's a literal phrase used to describe someone who performs well academically.

Here's why:

* Idiomatic expressions have meanings that are different from the literal meaning of the words they contain. For example, "kick the bucket" means "to die."

* "Good student" simply means someone who studies hard, gets good grades, and is generally successful in school.

While "good student" might not be an idiom, it is a common and widely understood phrase.