Bechoros(46a)/Gittin(36a)/(Makkos 16b): There was a case of a teacher of children who had physically abused his students [by beating them excessively – Rashi Gittin 36a] [ and some of them had died – Rashi Bechoros 46a] and as a result he had been made to take an oath by Rav Acha not to teacher children anymore [Rashi – Gittin 36a]. However Ravina had reinstated him because there was no other teacher who taught as precisely or as well.
Tzitz Eliezer( 22:59.2): You ask regarding Rashi (Bechoros 46a) who said that the students didn’t die immediately after the teacher beat him, “I don’t understand. Even if the students didn’t die immediately from the teachers blows, how could Ravina reinstate him just because he couldn’t find a better teacher – is it permitted to reinstate a teacher who causes his students to die? Furthermore you note that it doesn’t mention at all that the teacher promised to stop his evil ways?” It appears that this is a very serious question. In answer there are two issues 1) Rashi doesn’t have to be understood that it was typical that the students were hit harder than necessary that they eventually died but rather that it did happen. 2) It is also clear that after the teacher was removed from his position he repented of his evil ways. However he was not able to be reinstated because of the oath that Rav Acha had made until Ravina came and released him from the oath because there was need for this teacher and thus it was considered a mitzva. But this was only because he had already promised to turn over a new leaf.