I would probably check with an expert. I've never considered the label 'selective mutism' but it might describe my son particularly when he was 11-13. At 15 he is now coming out of his shell, but I did get him into a social skills class in junior high and he does see a psychologist now. He would tell me, why should I speak I have nothing to say.
Basically my son often wouldn't talk he didn't know a person well even if they were directly talking with him. This included kids his own age and adults. When I would ask he would tell me he had nothing to say. I had to actively work with him about the fact that people consider it RUDE if you don't respond when they are talking to you. And work on what is socially acceptable. He will do this in a class particularly if he doesn't know the teacher well. Last year he had a substitute teacher send him to the office because they thought he might be sick because he wouldn't respond. I've had him evaluated to see if he is on the Autistic Spectrum & that doesn't quite fit him, but the difference is it's not that he can't respond but that he chooses not to.
Good Luck.
