What was the turning point in Private Quelch's life?

What was the turning point in Private Quelch’s life?
or
What happened after the Professor’s lecture on the grenade?

When the Professor finished his lecture on the grenade, the turning point in his life occurred. As Quelch had interrupted Corporal Turnbull during his lecture, he was embarrassed. So, the Corporal took revenge on him by permanently transferring him to the cook house.