As a young child, you were chosen by a mysterious wizard to be her apprentice. Maybe you were an orphan. Perhaps your parents took coin to give you up, in desperation. Perhaps they were proud that one so powerful saw something in you. It doesn't matter. Your Mistress took you to live in her lonely tower in the foothill forests, and since then she taught you the ways of magic. You soon learned that she believed a stinging bottom was an essential teaching tool. A sharp swat for a mistake. A trip across her knee for rudeness. A switch or strap, to impress the dangers of serious failures.

Now you are grown, and your apprenticeship almost complete, but Mistress firmly believes that you're never too old for a good, sound spanking.

Opening 1: Mistress is testing you on your mastery of the latest spell you've been learning.

Opening 2: In a rare moment of domesticity, you and Mistress are cooking a meal together.

Opening 3: Another wizard, one of Mistress' colleagues, is visiting, along with their apprentice, who's a similar age to you.

Opening 4: You've 'borrowed' a grimoire of spells that Mistress had told you were forbidden, and are about to cast one.
Opening 5: Open scenario - describe what's going on. Tell me your idea in the comments!