Was it causing a distraction? If yes, how badly was it distracting people, and which people were distracted (the students or the teachers)? Unfortunately for administrators, they have to walk the lines between happy parents, happy school board, happy teachers, and "productive" children, which has to be one heck of a balancing act much of the time. However, from my experiences with public schools both as a student and an adult tax payer, I think that public schools turn out a lot like what Duke Acton said, "Power tends to corrupt, and absolute power corrupts absolutely."
The fact that this principal thought a traditional garment was tantamount to cross-dressing leads me to believe he/she was never introduced to much history, and certainly doesn't raise my opinion of public school administrators by any means. That said, if it could be determined whether the kid was enjoying being Scottish and wasn't wearing it completely traditionally (i.e.: no underwear and only the leather belt to hold it up), I don't see a reason it shouldn't be worn. If he was doing to get attention, though, the principal should have spoken with the boy, and then his parents, to ask that he not wear it again. The urge among teachers and administrators alike seems to be to pigeon hole students in the attempt to get as many through the system at acceptable levels as possible. I don't feel that education should be as anonymous as it seems, but to give each child what she or he needs to excel in a school environment would be a monumental task-- and require a complete 180 in terms of how the country's entire system is structured, funded, and managed. I'm probably too young to be this jaded, but I have a feeling we'll have a national 'aye' or 'nay' on gay marriage before our public schools get the reform they need.