In the poem's ironic final line, the teacher smugly declares
"Let that be a lesson” to a room full of “the dying and the dead”.
At the start his voice was "lost in the din" of the student's chatter, now his voice falls on ears that literally can't hear at all.
The teacher's outburst of violence may have been satisfying for himself, it didn’t do anything to improve students’ ability to learn - they're dead now.