The responsibility of a Rebbe, learned out from מחיית עמלק

The gemara in Bava Barsa (21a-b) has the following story (my rough translation):

Yoav killed all the male Amelekites. Dovid Hamelech asked him why did you kill only the men? Yoav answered because the pasuk says "Timcheh Es Zachar (the males of) Amalek". Dovid answered, no, the word is read as "Zecher (the remembrance of)"! Yoav answered, I was taught 'Zachar'! They called Yoav's Rebbi, who said that he taught זכר (since the gemara has no nekduos we don't know if he answered zecher or zachar in fact this is a machlokes). Yoav drew his sword to kill him based on the pasuk "Arur Oseh Meleches Hash-m Remiyah". Some say, he killed him; others say, he did not

What did the Rebbe teach Yoav and why was he blamed? Tosafos (21a) as well as the Rashba understand that the Rebbe taught Yoav "zecher" correctly. If so, why was Yoav ready to kill him? What did the Rebbe do wrong? The answer is that the Rebbe needs to not only teach the students correctly but he needs to ensure that they actually know the material correctly. Since Yoav left the with the incorrect p'shat of zachar we blame the Rebbe.

The Maharsha disagrees and understands that Yoav's Rebbe taught him "zachar" and his sin was that he became a teacher without first learning enough to teach correctly.

According to the Rashba we see the tremendous responsibility that a Rebbe has. If a student leaves the class with the wrong p'shat even if the Rebbe explained it correctly, the Rebbe is blamed. The Rebbe needs to make sure that every student gets the correct p'shat.