According to a study released on August 17 by the NFHS, high school baseball drew twenty-million fans during the 2009-2010 school year, trailing basketball, football and soccer.  The NFHS methodology was sport-specific, combining figures for sports for which both girls and boys teams were fielded.  

Complete story:  http://www.nfhs.org/content.aspx?id=5732

Update: The link to the original release now leads to a blank page.  Further information can be found in the story 510 Million Fans Attend High School Sporting Events in the September 2011 issue of High School Today: 

http://www.nfhs.org/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=5967&libID=5989