A few comments.
I didn't realize it was going to be a struggle to understand what I wrote!
The difference (regarding grammar) between the states, if any, is far, far less than the difference between the U.S. and Britain.
Being American, "School is out" makes perfect sense to me. I have no idea what the British say, but from the context it seems you are saying that the correct form is "School are out", which sounds like the speech of an illiterate to the ear of an American.
You must admit that had your brother gained his Masters in the U.S., his opinion of what is grammatically correct would be somewhat different. Had he gained his Masters 250 years ago, his opinion would be different again. (This is, in effect, my entire message.) The question is: Who has the authority to say what is correct and what is not? Presumably those who are keen observers of how educated people around them speak and write.
Neither you nor I have any control over the speed with which the language evolves, so why fret over it with "should"s?
Consider this: No matter which solution you settle upon ("is" or "are'), there will be some reader of your correspondence who will find fault with the grammar, because what is "correct" seems to be uncertain at this stage. Some readers will insist they are correct in thinking one way; others will insist that the other formulation is the correct one. You can't exactly quote your brother as an authority in a footnote every time you need to make that "is"/"are" choice, can you?