How does the schoolmaster Kantorek refer to his former students in All Quiet on the Western Front? Why do Paul and Kropp scoff at the term Kantorek uses?

The schoolmaster Kantorek refers to his former students as "Iron Youth."
Paul and Kropp scoff at the term Kantorek uses because it is incongruent with what the young soldiers have become. The word "iron" conjures up images of strength and durability, while the word "youth" conjures up images of innocence and life. Yet, Paul, Kropp, and the other soldiers are no longer the innocent, trusting youth they once were. Many of them have been physically and emotionally shattered by their war experience. The brutal violence of war has dulled their faith and enthusiasm for pretentious labels.
Paul maintains that it was men of the older generation like Kantorek who taught him and his peers that "duty to one's country is the greatest thing." To Paul, the older generation were "very free with all these expressions" of duty and patriotism. However, they were never the ones who had to sacrifice  life and limb to fulfill the dictates of their philosophy. In other words, Paul and Kropp think that the older generation has betrayed them.


They surpassed us only in phrases and in cleverness. The first bombardment showed us our mistake, and under it the world as they had taught it to us broke in pieces. While they continued to write and talk, we saw the wounded and dying. While they taught that duty to one's country is the greatest thing, we already knew that death-throes are stronger.

The horrific experiences of war have essentially made the young soldiers old before their time. They no longer stand upon the threshold of life, as Kantorek would have maintained. Instead, adrift on a sea of confusion and pain, they are bereft of any hope for a better future. Both Paul and Kropp are powerless as they watch their fellow soldier, Kemmerich, die. They can do nothing for him; all the patriotic catchphrases are impotent in the face of youthful death.
While the older men have "wives, children, occupations, and interests... a background...so strong that the war cannot obliterate it," young men like Paul and Kropp have nothing to hold on to. So, the idea that they are "Iron Youth" is ludicrous to them because it is incongruent with what war has reduced them to.