I am 12yrs old and I am racing the mile at league finals in twelve days. I have run a 6:05 but need to run at least a 5:53 to win. Is it possible to improve with only 12 days left? What should I concentrate on, speed, endurance or both?
With limited time left, you need to focus on speed more than endurance. It is very possible to drop 12 seconds in two weeks, especially at your young age.
Also, make sure you have a good race strategy going in to your mile. Here's a video that might help you come race day.