"There are sixteen and only sixteen students high school students participating in this school life of mutual killing. Those sixteen students are also the only people to have set foot in this school since we started this game." That's what Monobear said. As for the headmaster...

[To her credit, she manages to maintain her blank expression.]

He was executed before our game began. Enoshima left his body inside the school — that's why Alter Ego-san thought it was likely he was still inside.