Formal:
* Être un(e) bon(ne) élève - This is the most direct translation and is appropriate for formal settings.
* Avoir de bonnes notes - This means "to have good grades" and is a more specific way of saying you're a good student.
* Être un(e) élève studieux/studieuse - This means "to be a studious student" and emphasizes hard work and dedication.
Informal:
* Être un(e) bon(ne) élève - This is still appropriate, but you can also use:
* Être bon(ne) en classe - This means "to be good in class" and is more casual.
* Être doué(e) - This means "to be gifted" and implies natural talent.
For a specific subject:
* Être bon(ne) en... - Followed by the name of the subject, like "être bon(ne) en mathématiques" (to be good at math).
The best way to say "are a good student" in French depends on the specific situation. Choose the phrase that best suits the context and your level of formality.