And the fact that the principal is checking at all suggests that "easy access" has already become a problem.
At any rate, the lady's right: that kind of clothing has no place at a public event for kids.
Do you have anything other than anecdotal evidence to suggest wearing a thong will lead to "easy access"? I would suggest the wearing of a thong is unrelated to actual behavior.
And the fact that the principal is checking at all suggests that "easy access" has already become a problem.
Nice circular logic. "The practice exists, therefore there must be a reason for the practice to exist."
At any rate, the lady's right: that kind of clothing has no place at a public event for kids.
The question remains: is it the role of the school to enforce underwear codes, or the role of the parent to do so?
But is it right for her to do public inspections?