I often deliver school training on selective mutism and something I frequently hear is:
"It really is unbelievable how (he/she) can have so much control - to not speak for the whole day..."
My response to this is as follows:
"I can see why you would think that they are maintaining control by not talking but actually this is not the case. A child with selective mutism wants to speak but they can't.
Their condition is paralysing them to the extent that they simply cannot break out of the silence. They are not controlling this.
Imagine stage fright, where you freeze and cannot speak, but in every day life - this is nothing to do with control.
A child with selective mutism wants to speak to their teacher and friends - but it is just too scary for them to start. This is why it is our job to break the situation down into manageable steps for them."