* "Jour d'école" - This is the most literal translation and is the most common way to say it.
* "Journée scolaire" - This is a slightly more formal option, meaning "school day".
* "Cours" - This literally means "lessons", but can be used to refer to a day at school.
You can also use a more specific phrase depending on what you want to say:
* "Jour de classe" - Day of class
* "Jour de cours" - Day of lessons
Ultimately, the best way to say "day at school" in French depends on the context of your conversation.