Teacher Mu wasn't unwilling to deal, nor was he too lazy to speak; he was simply frozen solid by Gao Yi's words.
He was shocked, numb, his mind and body numbed together, from the top of his head to the soles of his feet - that kind of total paralysis where for a moment you can't speak or move.
Usually, extreme fear, extreme joy, extreme sadness, or simply a deep disbelief can cause this psychological lock-up.
I don't know which one its was for Teacher Mu, but snapping out of it was simple: shake him awake, shout him awake, or scare him awake.
Teacher Mu was clearly of the latter variety; still in a state of paralysis when he suddenly heard Gao Yi say it wasn't necessary, the effect vanished, and fear shot up. It was like a switch flipped back in his head, 'click', and his whole body lit up with electricity, then he could speak again.
The evidence was very clear, because Teacher Mu even started swearing.
