It can be done, but it's extremely painful. They basically break your jaw, then slowly force the pieces apart over a period of around 6 months. I had this done when I was in 7th grade, before I got braces. It was extremely painful at first, then kind of settled into a dull ache. There were 3 screws on the bottom and 1 on top. Made talking & eating very difficult (the hardware across the roof of your mouth makes it hard to swallow and enunciate clearly).

It was either this, or they'd have had to pull adult teeth to make room for the remaining.