Wrong approach. Every guy in HS thinks they just have to be tough. It doesn't work that way in races. What you really have to be is fit. Being tough isn't going to let you run faster than your current fitness.
So you think your current fitness is better than 21:00. Let's say it's 20:00 for a 5K.
Are there hills on the course? Is the course dry or muddy? OK, let's say it's flat and dry. Go out at 6:26 pace for the first mile. Not faster, not slower. See how you feel after a mile. Are you in pain? You are not going to break 20:00 today. You need to get fitter first (more mileage, more tempo runs, more 800s, whatever).
But if you're feeling OK after the first mile, then try to sustain that pace for the second mile. Stay relaxed. Don't push the pace. Let the pace come to you. If you're going to push and fight through pain, save it for the last 1.1.