One of my favorite books to read when I was a classroom teacher was Teach Like Your Hair's on Fire by Rafe Esquith. This inner city elementary teacher had some of the greatest (and funniest) lessons. The mask making comes straight out of his book.
What you need:
-Plaster Cloth cut in 1in x 4in you can get at any craft store or at Dick Blick
-Bowl of warm water
-Towel & paper towel
First off, put vasaline on eyebrows and eyelashes to prevent them from being ripped off.
Then, have a kid lay flat in a table and put straws in their nose. Dipping one strip at a time in the warm water, ringing it out a hit and placing them on the kid's face, it is best to have an assistant that will wipe away excess water that can drip down into ears. Cover the face in several layers of strips.
I've found it best to smooth out the entire mask with your fingers. This will make painting easier later on.
Once they are dry, have the kid wiggle their face a bit and pull the mask off. They won't be completely dry for several more hours so have a newspaper wadded up to set it on.
After a day or so have the kids paint the mask.
And after we snapped this, the cheesy lines started rolling like...
"I forgot to smile!"
"Wait, I think I may have blinked!"