Hi C.,
I am a speech pathologist and I have worked with 3 children who have been diagnosed with selective mutism. 2 of them "outgrew" it and it became an issue of "please stop talking right now." :) The other child actually used it more as control as she was in foster care and came from a questionable home life.
I talked to a social worker when i had these kids and he said forcing them to talk would be like sticking a snake in my face over and over, telling me that it won't hurt me or bite me, forcing me to touch it even though he knew that i was terrified of snakes. What worked for me was to work with the kids in small group at school. I set up communication where they could gesture or use pictures to get what they want until I gained their trust. Then as they were more comfortable, I made it a requirement that they at least say yes or no and built up from there. Each child is different, and I don't know what you have tried, but I might talk to the speech path at school to see if they can help.
Sorry I am not much help. It is a very difficult thing to deal with, but from what I have read, it is an anxiety disorder. My daughter also has anxiety and we are getting ready to trial her on some meds as well. I would think that it would at least be worth a try as they are probably just as miserable as you are.
Good Luck,
C.