Great improvement! For ANY high school girl who improves a lot in high school, the potential to continue after high school is great. SO many girls are superstars in middle school/junior high and then get worse or just don't improve in high school.
1) Up your mileage slowly between now and college (and yes, as of now D3 or NAIA is in your wheelhouse...and YES, you will be able to run in college even with a 25 minute PR (however, I think you will get better still)...not at every school (the top DIII schools don't have room for you unless you improve a lot), but a ton of DIII colleges will eagerly take runners with 20-25 minute 5K PRs. Emphasize your love of the sport and your improvement when you contact coaches.
2) Maintain a healthy weight...not too much, not too little. Talk to your doctor about this if you need to.
3) Make sure to get new shoes often enough. This prevents injury, and it is one of the biggest mistakes that high school athletes make. I realize they cost a lot of money, but they are worth it. Put 400-500 miles on a pair of shoes, OR you notice little nagging pains, OR you wear the soles out quickly because of how you run, then get a new pair.
4) For DIII and NAIA, especially for someone with your relatively slow times, you will need to contact the coaches. Email address are easily found on their websites, usually on the Roster page (typically a link to coaches there). Look at their rosters online, and get a sense about whether you think you can run there or not. Obviously, if all the women have PRs of mid 18s in HS, then you likely won't be terribly needed or wanted. BUT, there are a TON of very good academic DIII schools with small rosters who would welcome you. Again, emphasize your love of running and your improvement thus far. MOST NAIA schools are religious schools, so know that. That's a great fit for some people but not all.
Good luck to you.