Most common:
* Je suis étudiant(e). (Formal, but commonly used in informal situations)
More informal:
* Je suis élève. (Literally means "I am a pupil," but can be used for students of all ages)
* J'suis en cours. (Literally means "I'm in class," implies you are currently studying)
* Je fais mes études. (Literally means "I am doing my studies," implies you are actively pursuing your education)
Very casual:
* J'suis à l'école. (Literally means "I am at school," implying you are a student)
Important Note:
* "Je suis étudiant" is used for male students, while "Je suis étudiante" is used for female students.
* The level of formality depends on the context and the people you are speaking with.