Well I'm on my school's cheerleading team, and we do comp as well as sideline cheerleading, so this is what I know
Firstly two of the most well known high school cheering teams are Dunbar (watch cheerleader nation to learn more about them ), and greenup county ( read "We've Got Spirit: The Life & Time's of America's Greatest Cheerleading Team" by James T. McElroy, to learn more about them). I've read the book about Greenup, and watched all the episodes of Cheerleader Nation, both are very interesting.
A really good team in my area (greater boston), is Burlington High School. They scored a nearly perfect score (something like 96.5), at regionals, the highest every (I'm not sure if it was highest for our regionals, or or anyone's regionals). They are a town very centered arund their Pop Warner cheerleaders. There is a movie called "Blood, Sweat & Cheers", about the Pop Warner team competing, (similar to high school comp). My signature is from the movie! Burlington sells the movie at their competitions.
My School (Winchester High School ) is in Massachusetts, and we are all-girl however, if a boy wanted to, he could join. We compete on a flat mat,with no spring boards under us. We do basket tosses, and stunts, just like allstar squads do. We do dances in our routine, as well as a cheer. We also tumble. We cheer for Soccer, football, hockey and basketball.
Some of our girls come from All-star teams, most are from JAG and one girl is from East Celebrity Elite. Our town rival (also a fairly good cheerleading team ) Woburn has a coach who owns JAG so they practice and take tumbling classes there. Girls also tend to come from Pop Warner backgrounds.
You might have to ask a coach or ckeck rule books for certain things because I know that in some areas certain stunts are illegal.
If you have any questions or anything you would like to know about high school cheering (i don't know if you also wanted to know about sideline cheering or what), feel free to pm me!