No easy answer I guess. Big schools in middle school league are not allowed to move up 6th to 7th or 7th to 8th.I think you should play for the school you go to, but no matter what you do, someone will not be happy
I would think if I was a high school coach I would want a strong feeder program but if you take 8th graders to fill out your freshman team, you weaken your middle school teams
I do think if an 8th grader can play with the varsity team she should probably do so but when my daughter was a freshman she played varsity and came home telling us some of the seniors were talking about having sex, drinking beer and smoking pot, you have other issues also