For some strange reason, teams usually claim to be "National Champions" when polls rank them number one in the country at the end of the season that they played soccer. High School sports do not have true championships like some college or professional sports, just ranking in polls. Kinda like when they recruit football players and say that he is the number recruit in the nation, they don't mean that they had a competition for this title, just that some ranking organization said this person is the number one recruit.
These are the polls that I am aware of, but I am sure there are others.
NSCAA :
http://nscaa.com/hsr.php
Student Sports Soccer:
http://studentsportssoccer.Unapproved Website/
EurosportScoreBoard:
http://eurosportscoreboard.com/