It should be. Sensei was the one who was going to blow it up.

[Except....it's much more complicated than that. Nagisa's voice would be sarcastic if there were more life to it - he's tired of the government's little story, and knows he'll be stuck with the version they want to push forever. No one outside of Class E could possibly understand - no one outside of Class E would be willing to understand.]

It's easier to live with a comforting lie than a complicated truth.