As it turns out, I can indeed cite chapter and verse on this particular rule - because I literally wrote that rule in 1998. That's a big reason why people started calling me "By the Book".
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For reference, in the NFHS Rules Book, Rule 5-17-2b defines when the wrestling match concludes - when time expires at the end of the 3rd period (for a Decision). So, at the very instant the referee blows the whistle to signal that time has expired, the match is over, and there can be no more match points scored, and no more rulings which would determine who wins the match. I will leave it at that. The rule is very straightforward to apply.