When we talk about "are in session," we usually refer to a certain group of people, such as a court, school, or parliament, being currently open and conducting business. Antonyms for this phrase may include "out of session," meaning that the aforementioned group is closed and not conducting any official activities. Another possible antonym is "on break," which indicates that the group is taking a temporary hiatus from its usual work. Additionally, "adjourned" could be used as an antonym for "are in session," indicating that the group has ended its current meeting or term and is not schedule to reconvene again for a while.