She gasps dramatically, leaning in a little across the table so she's able to whisper to him - in a tone that is nowhere near a whisper's volume, mind you. People at the next table can probably hear it too.

"Are you, an upright professor, suggesting an innocent teenage girl like me should do that?"

Which isn't truly meant in some scandalized way (that's just for dramatic effect, really) as much as she seems kind of giddy at the idea of him encouraging bad behaviour.