After school

[SMEngmann]

Agreed, don't wait until after college. College in fact is a great time to start doing HS ball because you make your own schedule and will have availability that you might not have when you start a job, so it's a great time to get your feet wet. Plus, it'll help pay some of your expenses and it's a much better side job than what any of your friends will have. Many a ref has paid his/her way through college officiating.