It feels like the puppet goes "off the wire" once it's found out by all the characters that he is possessed. Because they're not sure whether it's the kid or the puppet is possessed (or both), they lock them both in the classroom.
Side question: Why don't they bring Tyronne out for the curtain call? The show is technically all about him, but Steven Boyer abandons the puppet for the curtain call.
"Mostly, I loved the size of these people's emotions. Nobody has emotions this size anymore. Outsized emotions. Operatic emotions. Kushemski and Vanda are like Tristan and Isolde, they're Paolo and Francesca. Nobody's in total thrall like this anymore. Nobody's overcome by passion like this, or goes through this kind of rage." Thomas, Venus in Fur