It depends on where in the world you are. In Canada, probably not; in the US? Possibly.
Edited to add: It also depends on the nature of the image. If the person in question is the sole subject and the she is clearly identifiable that's one thing, and if she is incidental to the main subject and clearly NOT the intended subject, it may be all right.
If in doubt? Seek permission/get a release.
Very true, but posting people images on a personal web site isn't publishing. What constitutes publishing isn't cut and dried.Taking the photos is one thing, publishing them is quite another.
Sorry 1 more question, why doesn't the school yearbook need release forms for students that appear in the yearbook?