I work for a school district in the technology department. We clearly spell out in our usage agreements that everything created on district equipment is for educational purposes only, and not to be sold for profit by either students or staff. Since this guy is using a school camera, I think this might be the policy he's running into.
Perhaps, if it was written down and his parents signed it or there was some valid way for all parties to agree with the rule.
From his Flickr site: "At the end of the [Texas Association of Journalism Educators] class, I approached the teacher confused, and asked that because I was using a school camera, and using a school press pass, do I still own my pictures? She replied that I did."
If he were using his own equipment on his own time, I'd be first in line to tell the school to blow it out his ear. But if he's covering these sporting events as a member of the yearbook staff for the school and he's turning around and selling yearbook pictures privately for his own profit, then no, I don't think he should do that.
Being on the yearbook staff does not preclude him from doing things on his own. It isn't a contractually bound obligation. Unless the press pass bound him contractually (which would be odd for a high school), it's just and ID to let you into places where the public is restricted.
In any event, the principal (at least according to TFA) is being an ass. Instead of sitting him down and discussing this rather complex real world issue, he / she (?) threatens with blackmail and suspensions. Not exactly role model material here.