This is exactly why every photographer operating outdoors should have in his pocket a copy of Bert Krages one-page summary of the law: "The Photographer's Right." Here's a passage that applies directly to this kid's situation:
They Have Limited Rights to Bother, Question, or Detain You
Although anyone has the right to approach a person in a public place and ask questions, persistent and unwanted conduct done without legitimate purpose is a crime in many states if it causes serious annoyance. You are under no obligation to explain the purpose of your photography nor do you have to disclose your identity except in states that require it upon request by a law enforcement officer. If the conduct goes beyond mere questioning, all states have laws that make coercion and harassment criminal offenses. The specific elements vary among the states but in general it is unlawful for anyone to instill a fear that they may injure you, damage or take your property, or falsely accuse you of a crime just because you are taking photographs.
There's more in this little sheet worth being familiar with -- especially suggestions about how to handle the kind of situation in which the principal found himself.