Many thanks to Leah for collecting these many ways you can view the eclipse safely:
suggestions for safe viewing: http://mreclipse.com/Totality2/TotalityCh11.html
in the section on pinhole projection method, where they mention "two thin but stiff pieces of white cardboard" - paper plates are very good for this. remember to stand with your back to the Sun when using this. you can practice on a non-eclipse day.
another projection method, not listed in the article is: look on the ground under a tree. the spaces between the leaves form many pinholes, and you will see a whole bunch of crescents as the sun is eclipsed. makes a great photo! http://www.hartrao.ac.za/other/eclipse2002/pinhole.html
and here is another link I found that you can share with your students, including step-by-step instructions for making a pinhole projection viewer: http://www.exploratorium.edu/eclipse/how.html
I tried several different methods of pinhole projection, and found that the "two pieces of cardboard" is easiest to use and gives the best results - even better than the one with the UPS tube in the exploratorium article.
paper plates work well and are easily available, but I used box lids from a case of copy paper. or you can use the sides of a cut-up carton, or a shoebox and its lid, etc etc.
you can even punch holes in a pattern (e.g. I made one where the pinholes are arranged to show the number 2012) and when you use it during the eclipse - each pinhole will show a view of the eclipsed Sun. great photo-op!
if using a pinhole projector, it's a good idea to prepare it ahead of time, and try it out before the eclipse. you can try different size pinholes (anywhere from very tiny, to about 1/8 inch) and see what gives you the best effect. I made one with actual pinholes, and another with pinholes enlarged using a sharpened pencil (that's for the 1/8 inch). you don't need to use aluminum foil - I made the pinholes directly in the cardboard. (and if you make one in the wrong place, just cover it with a small piece of paper or posterboard.)
besides testing the pinhole projectors outside, I also tested them with a flashlight in a darkened room, to make sure that what I was seeing was an image and not just a circle of diffuse light projected through a circular opening. you can also test them by using them when the Sun is partly hidden by a tree or building, and then you can see that you are getting an image and not just a circle of light.