One person associated with the student council goes missing and another one is found in a coma, and nobody can say where the council president was when those things happened. It's no evidence of anything, but it's a good starting point. Also, if someone of Bakarina's status is in a coma, wouldn't they have a light mage have a look? Such a mage could probably remove the dark magic spell used on her. Assuming it was such and not sheer physical brain trauma, which would be pretty strange treatment of the main character.