500 miles over a period of ~11 weeks is a solid goal, although I think you could handle a slightly larger increase given that you did 450 miles last summer. It also depends on how ambitious you want to be in this next year and what your goals are, i.e. putting up performances that would put you in contention to be recruited by solid college programs.
I don't know how much you've run in the past few weeks, but if I was your coach, I would plan to get you up into the 55-65 mile week range in July and August. I tend to think that more often than not, the optimal amount of running for an upperclassman in high school is about an hour of running a day with a 90 minute long run on the weekend, which puts you in that 55-65 miles per week range depending on how long your workouts are. Some might view that as being on the upper end of what a high school should run, but if your keep your workouts controlled, you shouldn't be too concerned about serious injury.
Happy to provide more advice if you'd like, or to answer any other questions.