Shuta was alone when they closed the door in the auditorium, but for only that moment. The culprit entered the auditorium afterwards- from the balcony! The culprit, Arvin, entered the balcony from the second floor. One of them shot Shuta with a gun or something. The balcony does constitute the auditorium. Afterwards, Aimee opened the door to the auditorium and before everyone else arrived, she stabbed Shuta’s dead body with the sword.
Afterwards, Aimee killed Garovel and Medlin and played dead herself. Arvin checked Aimee’s body, him being the culprit wasn’t fooled by Aimee. He killed Aimee and thus when the other two checked the body afterwards they justifiably believed Aimee was dead.
Just like it was possible to enter in the auditorium through the second floor into the balcony,
there was a way to enter in the lecture room from Arvin’s room through the window, and just like going from the second floor to the balcony in the auditorium doesn’t constitute leaving The Swiss, going from Arvin’s room to the lecture room through the window also doesn’t constitute leaving The Swiss. Let it be known that unless you explicitly say that using the window to enter or leave in the lecture room is the same as leaving The Swiss, this theory is not invalid.