My town is big into high school football - it is the most popular of all the interscholastic sports.  
Yeah, it is popular here too. Does high school football differ from college football in any way?
I'm not 100% on that, but I do know they seemed to both start at the same time, in the late 19th century.
When did football start to become the most popular sport?
Well, in the late 19th and 20th centuries many of the colleges actually played the high school teams, so I'm guessing it started around that time.
Interesting. How many players play on a team?
They have 11, but the National Federation of State High School ASsociations actually establish the rules for high school football in the US - two states use NCAA rules though.