Students might have different reasons for being passive. For them opting out is not a problem, it is a solution.
The trick is to make opting out harder than opting in, so that opting in becomes the solution.
For those students who have decided that being passive is a benefit, being active has now become beneficial.
However, students who use passiveness as a defense, might feel exposed and be reluctant to opt in. It’s important to provide them with enough support (from you or their classmates), so that they too can have a positive experience.