<i>A melamed who does not perform well can be fired without notice because the damage he does is irreparable. </i>
What is considered not performing well?
The point is that it would wreck major havoc on the Chinuch system if Rebbeim can be fired so easily when they get older. That is a major Pseida D’Lo Hodder.
(The secular laws in the US also make it illegal to fire older workers)