For a while I got into the habit of doing really quick portrait sketches in my diary, just before I went to bed. I’d rip out photos of faces from magazines, tuck them into the back of my Moleskine, then pull one out, rather randomly, at night when I was tucked up in bed to quickly sketch. It was a great way to practice sketching, and it taught me a lot about the structure of people’s faces.
I love this idea of using a yearbook for a similar task. The downside for me would be that all the head shots were from a similar angle, so you get less of a sense of the structure of the face. But it’s still a great idea, particularly for getting the sense of the differences between peoples facial characteristics, and for creating caricatures, as RobotJohnny has done.