Life-studies are something that would be great for learning different facial structures. As some of the other people have stated on here, if you can get into any local life drawing sessions that would be greatly beneficial.
One way I studied different facial structures is I looked up various photographs of historical figures and drew from them.
Another one of my friends practiced drawing facial references from various mugshots he found online.
If you want to study from photographs of real people, I would try to look for as much variety in faces as possible. Draw from faces with ideal proportions, as well as from faces with less then ideal proportions.
As another person said, basing facial shape off of geometric shapes is also a good way to start, especially when you are are drawing a more cartoony figure.
Hereās a tutorial on some unique facial features that I found pretty helpfulā¦.
http://jeinu.deviantart.com/art/Unique-Features-Tutorial-Pt-2-154756158
http://jeinu.deviantart.com/art/Unique-Features-Tutorial-Pt-3-198515118