Who Invented Pull-Ups?

The school said if we wanted them to get Elias started on the potties they have, we needed to bring in some pull-ups (Man those diapers are expensive...and on a side note I think Elias may be allergic. He has some bumps appearing...sigh). What I want to know is how do you change a child who has a gigantic poop (when he only goes once a day, it's big), didn't make it to the potty in time and is wearing a pull-up? I have absolutely no idea. If I try to pull it down, poop goes all down his leg, onto to his feet and onto the changing table. Perhaps I get the scissors out and cut them off?