One thing I do miss about high school is those Friday night games.
I'm assuming you mean football?
Yes football lol it was very popular at my high school.
What happens when they win the championship?
When the two teams play off at the championship the winner would be ranked one of the top teams and it gives a good chance for the players to go off to play college level.
I see, and what are some of the rules of high school football?
Lie most schools they players have to have good grades and a certain G.P.A. to play on the team make practice and integrity. They also play as one in pep rallies and all games. 
Is it popular only in the United States or in other countries as well?
No It is popular in Canada as well as in Europe. 
Can you watch high school football on television?