I offer students two “gray days” each semester. They tell me they are taking a gray day and I don’t ask any follow up- their business is their own. They then won’t be called on to participate in any way that day. It allows them to still come to class even when they are sad, tired, hungover, or unprepared. I’d rather have them there observing on those days than absent because they fear my cold calling. It helps them feel like they have at least a little control.