Celestia/Taeko was the mastermind the whole time! You figure this out when she reveals she's not actually dead after her amazing execution.

Hello, I have returned to inform you, you'll never get out of this school, not even death will save you, and you'll all play my murderous game until. You. All. Die.

She smiles and drinks some her rose hip tea while the remaining 7 students with you included look terrified