She’s fighting with him--a mock battle, but a battle none-the-less. He expects her to hold back but she won’t because in a real battle, they won’t hold back.
The music is bounding in her ear, the blond hair falls to the side as she rushes forward, breathing hard. His vibrant green eyes stare at her a moment, as her fists collide with his face. The beast is in her, the slayer wants out. It feels the power radiating from the child, and Buffy finds that she can’t control it anymore.
Harry knows this, and he steps back, trying to dodge another blow but he’s only human and she’s more. He’s too slow.
She rushes forward, the wind ruffles her shirt. She’s no longer Buffy, and he’s no longer Harry. She’s the Slayer and he’s the Vampire so she goes in for the kill, feeling as her eyes narrow when he dodges.
She makes a swift movement, still breathing hard. Her hands keep reaching out, trying to touch him in ways that would cause him pain. The stone walls are there, providing shield and protection for the children inside… but can it protect the children from someone whose already on the inside?
The Slayer feels as the vampire gets slower, he’s getting worn down. Good, that made it easier to stake them.
Automatically she reached down her black robes, searching for a weapon she never leaves behind. The stake is in her hand now, and she rushes forward, pinning the vampire to the ground. She can hear him breathing, and the Slayer knows that vampires don’t breathe. Though most do, out of habit or to try and make her hesitate. It won’t work, not on her. The Slayer pushes the stake down into his chest, and suddenly Buffy emerges from the Slayer.
Her eyes are green again, and she stares at Harry. The boy is unharmed; she’d emerged just in time. Buffy licks her lips, and tilted her head to the side in an inhuman manner, “Boom,” she said breaking the deafening silence, “Your dead.”
Then suddenly, everyone is there, gaping at the student and the teacher. Buffy stands, holding a hand out to a exhausted Harry. "In a real battle, that stake would have gone through your chest." She doesn't let him know that it almost had, that the Slayer had gained control because she knows it won't happen. She sees in his eyes though, that he knows that had she not fought back, he'd be bleeding on the floor, instead of joining his fellow classmates as she instructed them to get out their text book.
Disclaimer: I own nothing but these words I write. Joss wrote Buffy and JK wrote Harry.