With teaching, why does one have to be wrathful towards you? If they didn't care, they wouldn't be bothered or couldn't care less about what you do. So obviously with wrath, there is a degree of care and compassion that motivates someone to be wratful towards you right?
Say for instance you go to school. Everytime you make the same mistake in your maths calculation or didn't do your homework.. And after a few tries on helping you, your school teacher can choose, either to put you in detention, scold you or not care less anymore. But they only do it to a certain extent. After a few tries, they give up. But one's guru.. one's guru will use different ways and methods to make you realise and improve. He/She will not give up, (unless you give up on yourself or really choose not to change), and when using the wrathful energy, it is motivated purely by compassion and care, otherwise why waste energy and be wrathful towards you risking his/her reputation and your trust? There is no need to risk that if the guru did not care.
As with what has been said, using wrath to teach gives faster results because students get scared and want to improve, notices what they've done wrong and not repeat it and of course trusts that the guru does it due to reasons beyond what we can see and imagine amongst many other factors. Like HH. Kyabje Zong Rinpoche, who was known to be extremely wrathful and strict. When Zong Rinpoche was the abbot of Gaden Shartse, Rinpoche brought Shartse to new heights of monastic scholarship and the epitome of monastic discipline. Rinpoche was wrathful but look at the results.. It is so because it was motivated out of pure love and compassion. Imagine if Zong Rinpoche didn't care.. it would be exactly like what the current abbot of Shartse is now achieving under his tenureship - nothing (sorry to say!).