Subtly let her know.

"Just a little something, Ms. Larson," you say, a look of feigned concern on your face. "I... think there's been people fooling around on campus recently. If you know what I mean."

"Oh, really?" Despite the playful laughter in her voice, you know your question has caught Nicole off guard. "Um... Well, you shouldn't go around accusing people of... that, John. Not without - "

"Not without proof," you finish for her, edging the mobile phone from your pocket. It's enough to make your teacher's face turn a shade paler, although she still maintains her friendly smile. You give her a meaningful gaze, enough for your Maths teacher put two and two together.

"And... what are you planning to do with it?" she asks timidly.

"I don't know. You're the teacher. It's your call," you shrug as nonchalantly as possible and nudge your cellphone back into your pocket, watching Ms. Larson's smile slowly fade away into pursed lips. She recognizes that you're the one in control at the moment, and short of making a scene there's nothing she can do about it.

"...I think we need to have a little talk," Ms. Larson finally says quietly, after half an eternity in silence. "Off campus."