I remember an old manga from years ago.. I don't remember the title but I remember some details.

A male teacher who was previously a soldier or a hitman who fought on a battlefield. When he begins his first day in the classroom, he hears some ordinary noises like pencil lead snapping that reminds him of gunfire. He reacts in a strange way that perplexes the students. When he hears the sounds again, he crashes through the door out of the classroom.

So what is the title of the manga?

Jesus: The scene you describe happens in the 2nd chapter.

Yes, you are right about the manga Jesus. It:is the title I've been remembering but have forgotten.

Thank you very much!!!!