You didn't quite know what you were looking at. It was a wooden box with a big hole in the middle and leather padding. There was a little twisty knob on the side of it. You could only stare at it puzzled, having absolutely no clue what it could be.
"You know what this is, Natsuki?"
She beamed at you. You just chalked it up to her being smug that she knew something you didn't. Good for her, you thought to yourself. Natsuki knelt down, messing with it. Her hands went to one side and with a decent tug, the top of the box was open.
"Why don't I just show you. I don't think you'd get it. C'mere and lay your head down."
Classic Natsuki insult. She patted the box where she wanted your head to lay, and you couldn't help but feel a little weirded out by the situation. But you were curious, so you did as you were told. You eased into the box and the dark walls came higher into your vision. Natsuki smiled wide down at you. Then, the top of the box came down with a thunderous CRASH!. Barely noticeable in the commotion was a small click. Instinct took over and you tried to sit up at the sudden movement, but the box held you in place. Natsuki was leering down at you now, smiling mischievously. You tried to lift the box up, but it wouldn't budge.
She stood up next and walked over the box and you, giving you a clear view up her short skirt to a pair of pink underwear. A blush came to your face, and you hoped she wouldn't notice. Natsuki was too busy savoring the moment though, considering she knew what would come next. She maneuvered herself so that she was facing your feet and, once satisfied, lowered herself quickly. She made sure to flip her skirt up as she came down, giving you just an instant of seeing her panty-clad ass descend to your face before you were wedged in soft, musky feminine darkness. Natsuki let her legs come to rest against your body, really sitting on you with as much weight as she could muster.
"MMMPPHH!"
Natsuki giggled cruelly above you and crossed her ankles. Your face moved the tiny bit that the box you were trapped in let it, but it was still firmly planted in Natsuki's crack.
"You should have seen the look on your dumb face right before I sat on it! I don't know why there was a smother box in here, but I think I'm going to take advantage of it."
She leaned over for a second to grab a book, freeing your face for just a second. You used this brief moment to try to plea with her, but weren't giving nearly enough time to get a coherent argument going.
"Natsuki please let me ou-mmphhh!"
You were completely ignored as Natsuki settled back in, adjusting so that your nose was centered right where she wanted it. Her small, perky cheeks kept their place directly on top of you. You didn't get much air, and the little you did was filtered from Natsuki's panty-clad ass crack. While you were below her suffering, she was leafing through the manga she'd picked up, ready to settle down and relax for at least a little bit.
What happens next?
1) Natsuki reads leisurely and loses track of time. Eventually, one of the other girls comes in to check on the two of you. How does she react when she finds Natsuki sitting on your face?
2) Natsuki decides to let the 'smother' box live up to its name. She giggles as she deprives you of air beneath her ass.
3) Something else.