Karen grits her teeth as soon as she hears the voice in her head, because she is not receptive to intrusions such as that, then she bows to the teacher and returns to her seat. As much as she tries ignoring the voice, she eventually succumbs to it.
[b][color=darkred]I kindly suggest you get out of my head before I find you and kick your ass seven ways to Sunday. And as for your question, no, I can't control my quirk. Now out.[/color][/b]
Perhaps to the rest of the class she'd seem insane, but she [i]kinda[/i] assumes they know who she's talking to, and that they'd be able to tell she's talking to him due to the amount she's grinding her teeth together. Also, nobody would really know her quirk seeing as though she hasn't shown it yet, all they know about her is her name and the fact she has flaming hair.