Of course she does

You hear a gasp from behind you.

"Excuse me!" It is Miss Wilson. Suddenly she is leaning over from behind you, placing her head in between the two of you. "Exactly what are you two doing, behaving that way in school?!"

Sarah tries to play dumb. "What do you mean Miss Wilson? We were just talking."

"You know exactly what I mean, young lady! I'll expect to see both of you after class." Before you can protest, Miss Wilson stops you. "I don't care if you do have another class right after this one! I'm going to have a little talk with you!"

Miss Wilson went to the front of the class to begin the lesson. What did she see, exactly? She was behind you, so surely she didn't notice Sarah flash you. Maybe she's not sure what she saw. Should you just play it cool and deny everything, or maybe even skip out after class? After all, if she tries to report it, then she has to explain why she had you stay after, and what could she have seen or heard?