Most common:
* À l'école: This is the most general and common way to say "in school." It literally translates to "at the school."
Other options:
* Au collège: This is used specifically for middle school.
* Au lycée: This is used specifically for high school.
* À l'université: This is used for university.
Example sentences:
* Je suis à l'école. (I am in school.)
* J'ai appris ça au collège. (I learned that in middle school.)
* Il étudie au lycée. (He is studying in high school.)
The best way to choose the correct phrase depends on the specific school you are referring to.