Formal:
* Il n'y a pas cours demain. (There is no class tomorrow.)
* L'école est fermée demain. (School is closed tomorrow.)
Informal:
* Pas d'école demain! (No school tomorrow!)
* On a pas cours demain. (We don't have class tomorrow.)
* Demain, c'est jour de repos. (Tomorrow is a rest day.)
You can also use specific phrases depending on the reason for the school closure:
* L'école est fermée pour cause de neige. (School is closed due to snow.)
* Il y a un jour férié demain. (There is a holiday tomorrow.)
Remember:
* "Cours" refers to classes, while "école" refers to the school building itself.
* "Demain" means "tomorrow."
Choose the phrase that best fits the situation and your level of formality.