Yeah she was definitely the most suspicious. If the powerful person that snuck into the school was to be anyone, it would be her
Edit: surprised reading the comments no one expected the teacher. First, she was looking for the iceblade magician from the start. Second, she’s “infiltrated” the school so has to be either a student or a teacher. Third, she’s the only teacher or student with power comparable to the iceblade magician. Fourth, they foreshadowed either her dying or something tragic happening like betrayal. Fifth, they made her likeable so her betrayal would actually mean something. Sixth, she’s a drunk which makes it easy to hide her infiltration. Seventh, the Eugenics sneaking in and harming students was done on her watch. Like I said, she was definitely the most suspicious. Sure we didn’t want it to happen, but the author didn’t pull this out of nowhere.