Personally, I'd beat my daughter's ass if I saw pictures like this when she was 15 or 16. On the other hand, though, the pictures were not pornographic, the girls were clothed, with private areas covered, and while definitely suggestive, I see nothing illegal based on what I know of the law.
However...if these suggestive pictures were being shared in a school setting I think the principal has the right to discipline the girls. The school system sets the standard for appropriate behavior, discussion, and imagery in school settings. If he had to search through their phones or cameras to find the pictures then there should have been no punishment (but perhaps a discussion with the parents of the girls).