How can I improve from a 23 min 5K to a sub-20 minute 5k?

Next year is my senior year in high school, and I want to have a really good cross country season. My personal best was a 23:54 for the 5k last Thanksgiving. I really want to improve to under 20 minutes by the end of my season in late October. Is this possible? Also, any advice on what training schedule to follow would be great.