There's probably an important lesson here: when someone is justifiably angry at you, a good way to soothe the situation is to see the big picture, acknowledge that the other person is right, and not argue about minutiae.
Why is the teacher so angry in the first place? Is a 7th-grader really such a threat to her? Does the district have an anger management course she can attend?
I mean, yeah, I get where you're coming from... we're all helpless to control the irrational behaviors of others, so the best we can do is adopt coping mechanisms for dealing with babies of all ages, and teach our children the same. But at the same time, this is sending a terrible message to the child, because other than exhibiting an age-appropriate misjudgement, by attempting an ill-advised prank, he has done nothing wrong here. Reinforcing the teacher's message, that it's his fault that she can't behave professionally, is not good for his psychological development.
His correct response (IMO) would have been, "I can see how my actions could be dangerous; I could have opened the door into someone who didn't know I was there when I came out," or "I could have knocked something over and hurt myself."
IMO, it's not his job to explain all the possible negative outcomes. He's a child. That's the job of the responsible adult, and if she's offering reasons that are just plain wrong, she shouldn't be doubling down on them, she should be accepting his information and offering better reasons, like the ones you gave here.
