Why do Paul and Kropp scoff at the term Kantorek uses for his former students in All Quiet on the Western Front?

Schoolmaster Kantorek refers to his former students as "the Iron Youth".  Paul Baumer and Albert Kropp scoff at the term because it is so ironic.  Kantorek speaks of them as if they are glorious and indestructible, but in reality, as they find out so quickly on the front, the young men are not iron but only flesh and blood, and their youth is stripped from them forever by the realities of war.

Paul speaks with bitterness about men like Kantorek, the older generation who are so enamored with high ideals of patriotism and love of country, and who so willingly send the young out to do the dirty work of war.  As a schoolmaster, Kantorek had used his authority to convince the impressionable "lads of eighteen" in his classroom that it was their duty to enlist to fight for their country.  Never did Kantorek nor any other of the boys' role models try to make them understand about the horrific reality of what they were about to undertake.  They taught that "duty to one's country is the greatest thing", but after a short time on the fields of battle, the young men learn unequivocably that "death-throes are stronger".  By that time it is too late to preserve their youth, however.  The things the soldiers experience rip away their innocence forever.  With the harrowing realization of what war is all about, Paul and his friends know that the term "Iron Youth" is patently false.  They are neither indestructible - "Iron", nor are they any longer innocent, or youthful (Chapter 1).