How can I train for a good mile time?

I'm in 7th grade and I really want to impress my cross country and track coaches and my hopeful future coach. It's snowy in my area so it's a little tough to run, but I've figured out a way(I wrap my sneakers in cellophane. If you have any better ideas or some pointers in running in the snow they're more than welcome). I need some good workouts that can't take a lot of time. I'm extremely busy on certain days and have to have a flexible(is that how you spell it?) schedule. Please use distances appropriate for my age. I don't want to be running 40 miles a day. Thanks:)