There was an incident a couple of years ago where a teenage girl during an art lessen got her hands stuck in a bucket with plaster. As the plaster set, the heat from the chemical process caused her fingers to melt (it says in the article that the temperature reached 60 °C; I am not sure how this can melt your fingers, but this seems to be the case). As a result, she had to have all but two fingers amputated. The lesson we seem to learn is that sticking your hands into a bucket with plaster is a bad idea.
At the same time, plaster casting the human body is something that people do all the time, both in art and for medical purposes. Of course, the layers are thinner in this case. But in this video (which is erotic, sorry about that, but it was the only video example I could find), a woman puts her entire body into a bucket of plaster and seems to get away with it just fine.
So, what are these people doing differently than in that school incident? Could they be using a different kind of plaster that generates less heat?