Take a look at Mark Kistler's Site- Draw3D
Here there are many videos and drawing tutorials that give step by step instructions that can result in fabulous drawing by even the youngest students! The videos are short, about 8 minutes or less, and usually take about 15 minutes to finish because you might have to stop them so students can catch up. The first couple times doing these tutorials might take a little longer, but once students are used to them, they can whiz through them!
I had to get the book that is pictured above for a class, and make sketches out of it. This would be another great classroom resource to have. There are step by step, drawn out examples of things to make- they are all silly characters that students love! The book also goes over the 12 Renaissance Words.
Happy Drawing!
-J