I went to middle school with a guy like this, Never once heard him speak in the entirety of the 4 years we were together, the teachers knew about his condition so they never called on him to read aloud or answer questions that didn't involve just writing the answer on the chalk board. He was oddly popular for someone that never talked and everyone that had class with him liked him.
I had a lunch period with one of his friends that knew him from outside school and apparently he talked like normal when he was in his own house.