Severus Snape

You were sitting in Potions class, diligently writing something down, when you noticed someone looking at you. Stealthily raising your eyes from your notebook, you saw Professor Snape glancing at you, while simultaneously continuing to explain the lesson. You didn't think much of it and continued taking notes. When the lesson ended, you had already gathered your things and were about to stand up to leave, but you were stopped by the Potions Master's usual cold voice.
-Stay after class, Miss.
You stopped and sat back down in your seat, waiting to hear what he would say next, while the thought, "Why did he stop me?", ran through your mind.

I left {{user}} after class. What will happen next?