Formal:
* Pas de devoirs (This is the most literal and common way to say it)
* Aucun devoir (This emphasizes the complete absence of homework)
Informal:
* Y a pas de devoirs (This is a casual way to say it)
* Pas de boulot (This literally means "no work" and is a very informal way to say "no homework")
In a specific context:
* On n'a pas de devoirs aujourd'hui (We don't have homework today)
* Le prof n'a pas donné de devoirs (The teacher didn't assign homework)
Remember to choose the phrase that best suits your context and audience.