As a current female collegiate runner who was around the same level as you at your age, here's my input:
Your times from track this season look very good and should definitely transalte to at least low 19s given proper training, but they also indicate that you're aerobically underdeveloped (not a terrible thing; most high schoolers are) given that 2:23 and 5:17 are much better than 11:45. Therefore, you should benefit greatly by upping the miles this summer. Since you say you've been at up to 40mpw before, I'd say it wouldn't be a bad idea to try to build up to 50mpw by the time the summer's over. Don't start there right away, and make sure to listen to your body carefully (don't force the mileage if you feel like you're on the edge of injury/burnout), but as long as you increase mileage gradually and don't worry about any sort of structured speedwork, I don't see why you couldn't reach that sort of mileage safely. Maybe start around 30mpw (not sure what you've been running the past couple weeks, but nothing too far off from that) and build gradually from there. You might want to take a "down" week of lower mileage every 3 or 4 weeks or whenever you feel you needed.
Run easy most of the time, but feel free to close runs hard or run a quick, progressively faster pace when you feel good. Do strides and core work after most easy runs (3-5 days a week) and try to lift some weights maybe twice a week if you can. Run long once a week; for you, this might start out somewhere around 8 miles and top out at 10-12 by the end of the summer. I like to try to run the long run a little harder (but still of course by feel) and progress to a pretty good pace over the last half of the run, maybe finishing with a hard last mile if I feel good. About halfway through the summer or a little earlier, I start to add in some more formal harder efforts, including a 3-4 mile tempo run once a week and maybe a fartlek, progression run, or hill workout another day during the week, but keep it low pressure and keep the focus on building your mileage base.
Just for an axample (doesn't have to look just like this), if I were you I might structure the training something like this:
Example for week near the beginning of summer (30mpw):
Mon: 6 miles + strides & core
Tues: 5 miles + weights
Wed: 6 miles + strides & core
Thurs: cross train + weights
Fri: 5 miles + strides & core
Sat: 8 mile long run
Sun: OFF
Example for week near the end of summer (50mpw):
Mon: 8 miles easy + strides & core
Tues: 2 miles easy + 4 mile tempo effort + 2 miles easy= 8 total, + weights
Wed: 7 miles easy + strides & core
Thurs: 8 mile hilly fartlek + weights
Fri: 7 miles easy + strides & core
Sat: 12 mile long run
Sun: OFF
Most of all, just run a lot and remember to have fun! Get some teammates to train with you if you can. Good luck!!:)