and (Quick! Everyone pretend to look surprised!) it's this student.
The evidence isn't rock solid, but I'm certain enough. (Two pieces of terminology used wrongly and weirdly and one made-up term that doesn't even exist. Both identical to the mistakes of the girl who was sitting next to her. None of these particular mistakes appeared in any of the other 88 students' quizzes.)
I'm not sure what I'll do if she doesn't confess, though. Even if I could prove that the two quizzes are too similar for it to be coincidence, it could have been that the other student copied from this one rather than vice versa. Considering the circumstances, I don't think that's likely, but I can't prove it isn't so. And I can't penalise both students for something that one of them was probably unaware of.