If you can, buy or borrow a well made medical or artist's model skull to draw from - beware of inaccurate Halloween decorations. All tertiary art departments should have their own skeleton, and a highschool science department will have one. If studying independently, Molded plastic skulls are available from some art suppliers and medical equipment suppliers. (Photographs are a last resort, but better than nothing.)
Your model should preferably be life size, as it will help you have a clear sense of the relationship between the skull and the visible surface anatomy of the head. Check that the jaw is correctly placed, and if using a full skeleton, that the skull is correctly placed on the neck.
If you can't access a real skull to draw, you can still benefit from copying good photographs. Try to use images showing the skull from various angles so that you can build up a three-d picture in your mind.