As a middle school student, you can play any level of high school sports including varsity. You can then go to any high school and be eligible when you enter 9th grade.
The only restriction here in Jefferson county is that, as a middle schooler, you have to play for the high school that is your "resides" (or home) school. you cannot simply pick or choose what high school you play for. Even if you have no intention of attending that high school, that is where you must play.
That is why you're not penalized when you get to 9th grade and end up playing for a different high school.
I've had first hand experience with this rule, unfortunately.