Nadie,
I understand why you objected to being characterized as a racist in the exchange you described. It was silly of the principal to do so. Are you saying that there is nothing that can be accurately described as racism out there. If I hate you because you are Latino, or black, or white or Asian or whatever, why isn't that racist?
I just would steer clear of calling a Catholic, a racist. Give them the benefit of the doubt. Don't jump to conclusions.
A kind of analogous technique that I use when people get violent with me (face to face), is to act as if I don't understand what they are doing, usually, they will just leave me alone since they see that I'm not scared at all. But if they are really intent on harming me, and anyone is watching, after hearing me be verbally abused for so long, the witnesses will testify that they would have acted the same way as I (break a broom over their head), LONG before I did. Actually in 30 years of using this technique the result has always been that "they will just leave me alone since they see that I'm not scared at all", so don't conclude that I'm some bar room brawler.
Give them every opportunity to explain themselves, ask them all manner of questions. If you finally conclude that they really are racist, by that time "the witnesses will testify that they would have concluded the same way LONG before you did".
The biggest source of a mis-diagnoses of racism in Americans is that they don't know how to differentiate class difference from racial difference. It is the biggest sources of error. Scarcely any Americans realize this. Class difference is the source of "discomfort?" (nothing in common?) between people, not race. A upper class white baptist would be more at home with an upper class Latino, than he would be with a lower class white baptist. The worst scenario of "discomfort" would be the combination of an upper class white baptist with a lower class Latino, and similarly an upper class Latino, with a lower class white baptist. The "discomfort" is mutual between upper and lower classes. The lower class Latino does not feel comfortable around upper class Latinos, just like a lower class white baptist would not feel "at home" with an upper class white baptist.