You are almost to the bottom of the stairs when you start to feel a horrible throbbing in your rear. It starts as a tiny pain at the cleft of your butt but it soon becomes a sharp stabbing pain. With each stabbing pain you feel an urge to push your rear back. Your heart is pounding as you struggle down the steps. The pain soon spreads to your entire rear. It feels like you've been spanked for hours by the time you reach the bottom of the stairs. Above you you hear the sounds of boy's voices being forced into brays.
After what seems like hours but is really only a minute you start to feel something pushing at the back of your pants, only slight at first and when you loon back you can't see anything. Feeling with your hands soon finds the hard lump at the base of your butt pressing to behind you. You stagger away from the coaster as it grows bigger and bigger stretching the back of your pants till its visibly sticking out behind you tight as a drum. Whenever you you lean forward you feel the cloth tighten.
A minute later an urge makes you bend forward and with a loud ripping sound your pants split down the middle. Soon after the pain is gone but looking back you find a long donkey tail sticking out of the ragged hole now in the back of your pants.