In general cross country speak, varsity is usually a team's top 7 runners. JV is everyone else. So when it was mentioned that Jayme wasn't always listed in race results because she was probably JV, just means that she wasn't in the Top 7 on her team. Many meets only provide official results for the "varsity" race, especially at the middle school level, although some middle school races are "Open" and everyone runs in the same race. I think it is pretty much the norm for all grades to compete together in middle school (though in IL, they separate races by grade). Some states (MN and NM for example) allow middle school athletes to play for high school teams, but I do not believe that WI is one of them. There is usually no specific age determination for JV or varsity. It really comes down to who is fastest. My MVP this year was a freshman (obviously varsity) and I have a 4 year Sr who was JV. It makes team selection very easy. As we like to say, "the clock doesn't lie".
So really, the answer is that it varies...
Varsity in MN is high school where I lived. JV were kids not good enough in high school to be on varsity. Unless there was a standout middle school kid. Then they could be JV. I don’t think Jayme was JV. She does not sound like she had the top ability.
States have different amounts of money. And different focuses on sports.
In MN where I lived, no one cared about football as a school sport. But hockey! Kids were on traveling teams at age four. That means traveling to other cities.
I think Wisconsin is football because of the Packers, a publicly owned team.
I have no idea of the running and other things such as the hurdles, etc