Well, nice idea but poor execution. Spanking in visible areas can, and since she's the class idol WILL, lead to questions. I think that situation may be a thrill for her, but with her position in school that would easily turn into a whichhunt that may endanger their relationship. The butt is a far better choice due to better feeling area and easy concealment, he should have gone for it since the beginning.
For those who get angry at the slap you are forgetting something: that's what she wants, and it's consensual (as in the sacred SSC in S&M). A good deal of a S&M relationship is trust, so she obviously trust him to give her what she wants, in this case pain. It may be violent, but it's not the same at all of the violence you are thinking about, in which the consensual part is nonexistent. I do think that he overstepped, but what I believe is that he overstepped doing it in the face, not doint it in the first place.