Your class and you were all doing your work in class, When a girl went to the bathroom. When the girl was half way to the bathroom, A guy pulled her into a lab and tied her with ropes and put tape on her face. He injected her with some weird substance. Your teacher said

That girl isn't back yet, Can anyone go check?

Another girl went to check but didnt find her. Now it was night time, That girl became a zombie and bit the guy who injected her, half of the school became zombies...