So I quit track/running but have some questions

I've never heard of a policy like that before, and I bet it doesn't actually exist at your school. Especially at a public high school, it seems like something that would be difficult to maintain. If necessary, have a parent talk to the coach, athletic director, or principal for clarification.

But it probably won't be necessary. Write an e-mail to your coach in which you say,

"Dear Coach, I'm sorry I didn't explain my decision to quit the team very well. I saw a doctor about my knee problem, and he advised me to pause my training and get physical therapy for now. I'm taking my doctor's advice because I want to be the best runner I can be. I plan on taking my training seriously this summer as soon as I'm healthy so that I can make the greatest contribution possible to the cross country team in the fall. Sincerely, You."

Adjust the details as necessary. Basically, try not to sound like a whiny, rude, unreliable flake. Have someone check your e-mail for tone and spelling before you send it.

Then go get healthy, then go train smart, and then go listen to your coach.

^^^^Do this.^^^^

Don't embellish the email. Tell your parents this is what you are going to do. Send a copy to your parent(s) email address. You could copy the athletic director at the school, maybe the principal if it's a small school.

Depending entirely on how long you are doing rehab, see if you can find an alternative "knee safe" sport to keep some fitness, maybe develop some other interests you have? Waiting for something to heal can be a bit crazy making, so try to find something else that is interesting to you.

When you are healed and ready to go, show up at the start of the workouts/tryouts.