From the article: "The school system's dress code forbids disruptive clothing, grooming and symbols. Principals decide what's disruptive." So the principal made the initial call (I think we agree on that part). After that, I think the instinct of administrators is generally to uphold the principal's decision, for fear of undermining his authority.
Unlike others on this thread, I'm not doubting that the principal's action was "legal", and it may very well be upheld by the courts. It was still highly inappropriate and unprofessional, by all indications.
I've no doubt that the principal was wrong 6 years ago when he had his dui.
I also think that the court will uphold the principal. (I think there's a lot not said in this article.) In my opinion, the student also is out of line and distracting from the school's educational mission. That's just me.