How many students do you have? While 45 minutes is plenty of time to apply
plastercraft, have it harden, and then remove it, you'd have to have lots of
helpers to get an average size class done in one class period.
I now have the luxury of having warm water in the classroom but it will also
set up okay with cold water. I didn't notice much difference in setting
time between the cold and warm water.
I've used vaseline on their hands/arms to keep the plastercraft from
sticking too badly. I also used to use this when I was casting their faces,
but these days (trying to be a little safer) I use plastic wrap over their
faces. I've never had any adverse reactions to either the plastercraft or
to the vaseline, but I know that there can be allergies to it.
When you're casting hands, just remember not to put the kids in a complete
cast by going all the way around their hands/wrists with the gauze!! I
learned this the hard way! ;-)