18 flat is tough. It's about a 5:47 pace. That means you'd be running your pr for the mile for 3 miles and some. And it's not on a track.
Its extremely difficult and I wouldn't count on it. You'd need to cut your mile time down quite a bit. You need to get down into at least the 5:20's for the mile to run a 18 minute 5k.
Put in more miles. Make sure to ramp up though. Follow the 10% rule. Get up to 40 miles a week. Long runs of 10 miles about. That should be good for your age. And dont be stupid. If something doesnt feel good take a couple days off. I wish I followed this advice.
In 8th grade i had a pr of 5:45 in the mile. In 9th grade the best I could do in the 5k was 19:43. Of course I wasn't putting in a lot of miles. I only ran an 18:00 5k in 11th grade, and that was when my 1600 meter PR was under 5 minutes.
Don't worry if you can't make varsity next year. Slow and steady. You'll get to 18 minutes eventually if u train right.
And I'm not an expert. I'm just an incoming senior that was in your position a couple years ago. I've got PR's of 4:40 in the 1600 and 17:47 in the 5k. Guess those are my qualifications...
Good luck