DY:
The underperforming rebbi is a serious risk of being abusive. The good rebbi makes his teaching enjoyable enough to the talmidim that motivates them to learn. They behave, and progress according to their capability. The poorly performing rebbi is not doing that. He is apt to follow the lead, and utilize discipline to beat his students into compliance through strict reward and punishment. It will assist in classroom management. But it will fail him academically. One must assume the poorly performing rebbi is at high risk of becoming abusive, if just to maintain some semblance of control. One does not need to get to the abusive level of molestation to be damaging to kids.
M:
Your idea of tenure just formalizes what occurs anyway. Nice try. But this is currently out of the box. Today’s hirings are done by the connection with the hanhala, not merit. “It’s who you know, not what you know.” That is as painfully true in Chassidishe yeshivos as in non-Chassidishe yeshivos. If quality were the main ingredient, then training, which is available but underutilized, would give one applicant priority over another. Sadly, that does not happen. Meanwhile, once someone has managed to stay at a job for a while, he has seniority. Perhaps no formal tenure, but the same result.
I know my comments here sound quite cynical about chinuch. A word or balance is needed. It is wholly possible that a rebbi functions quite well for many years, and then slackens off. Aside from personal factors related the rebbi’s own personal life and aging, the population of students is undergoing great change. No rebbi I ever had was at risk of having his cell phone ring during class. Why? Won’t say my age, but it should be easy to answer that question. Kids today are living in an ever changing world, with new issues that did not exist when an older rebbi began working in chinuch. That’s a great explanation for the mandated continuing education that exists in every other profession.