An average high school player might actually have 2's and 3's (20 - 30). Keep in mind the rating system is based on MLB skills.
So comparisons are based on what a major league player would rate. Few high school players will have many skills that would be at an average MLB level. (40 - 50)
This article may help explain:
http://www.hsbaseballweb.com/pro-scouting/rating_system.htm