The way I understand it is this. If you are a regular 8th grader you can play jv sports. However, if you are a hold back 8th grader you are not supossed to play any jv sports. Once the holdback reaches the 9th grade he is elgible for varsity. As far as your question goes. If you play jv while you are a true 8th grader and decide to hold back you can't play jv again the following year. I am aware of one kid for sure that is in this situation. He played both 8th grade and jv basketball last year and decided to hold back and wasn't allowed to play jv.