I think this rule used to be broken all of the time, but now KHSAA is becoming a lot more strict about it and more and more JV teams have middle school players on them. A lot of schools would struggle to field a JV team without middle school players these days. That's sort of why I was asking about the NKY programs. The officials have nothing to do with this. It is the responsibility of the coaches.
FWIW, I think the rule is silly to go by grade. It should be by age. There are a lot of kids who are ahead in school or behind in school. So a kid who is a year ahead in school can play varsity as a 13 year old against 18 or 19 year old seniors, but a kid the same age can't play JV as an 8th grader against juniors (some of whom are probably sophomore age)? And what about those sophomores who are actually junior age? No rule would be perfect, but age makes more sense to me.