You can register as soon as you have the lease.

at any point before the school year starts / during the summer / at the very start of the year.

the earlier you move / register the more likely you are to find spots in desired classes

We are considering renting in-bounds for a W high school. What should our timeline be? When would we want to be ideally relocated to ensure that we could register our rising freshman for them to participate in orientation and class selection ( while still in 8th)?