As the first line of "The History Teacher" makes clear, the teacher in question wants to "protect the innocence" of his students. He does so by downplaying or using euphemisms to describe the cruelty and inhumanity that so often emerge during the study of history. He characterizes, for example, the "Ice Age" as the "Chilly Age," and tells his students that the Enola Gay only dropped a single atom on Hiroshima. It also seems that the teacher is simply unwilling to engage with the difficult work of teaching controversial or potentially disturbing topics, as the poem describes him walking home "past flower beds and white picket fences."
The poem provides some commentary on the question of whether teachers are justified in shielding students from the truth. Even though the teacher goes to great lengths to avoid disturbing questions, his students, the reader learns, go straight from his classroom to the playground, where they "torment the weak and the smart." So his approach of avoiding the difficulties of history clearly doesn't preclude cruelty on the part of the students in his classroom. In fact, Collins seems to suggest that the teacher misses the opportunity to educate his students about the world they live in. The students, not being require to come face-to-face with the injustices we see in history, thoughtlessly engage in cruelties on the playground.
These questions are very much current in education today, and there is no easy answer. An honest, rigorous study of history will always be difficult, and while teachers should emphatically not hide or lie about the past, they must consider a number of factors, most significantly whether the material in question is appropriate in light of student age and developmental levels. They are also ethically bound to teach in a way that is sensitive to the emotional well-being and the cultural backgrounds of their students. So while the history teacher in the poem is failing to educate his students, the issues raised by this poem lack simple answers.