High school football
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High school football is gridiron football played by high school teams in the United States and Canada. It ranks among the most popular interscholastic sports in both countries, but its popularity is declining.  According to the Washington Post, between 2009 and 2019, participation in high school football has declined by 9%. 
The National Federation of State High School Associations (NFHS) establishes the rules of high school football in the United States.
Since the 2019 high school season, Texas is the only state that does not base its football rules on the NFHS rule set, instead using NCAA rules with certain exceptions shown below.   Through the 2018 season, Massachusetts also based its rules on those of the NCAA,  but it adopted NFHS rules in 2019. 
With their common ancestry, the NFHS rules of high school football are largely similar to the college game, though with some important differences:
- The four quarters are each 12 minutes in length, as opposed to 15 minutes in college and professional football. (Texas uses the NFHS 12-minute quarter.)
- Kickoffs take place at the kicking team's 40-yard line, as opposed to the 35 in college and the NFL. (Texas has adopted the NFHS rule.)
- If an attempted field goal is missed it is treated as a punt, normally it would be a touchback and the opposing team will start at the 20-yard line. However, if it does not enter the end zone, it can be downed or returned as a normal punt.
- Any kick crossing the goal line is automatically a touchback; kicks cannot be returned out of the end zone.
- The spot of placement after all touchbacks—including those resulting from kickoffs and free kicks following a safety—is the 20-yard line of the team receiving possession. Contrast with NCAA and NFL rules, which call for the ball to be placed on the receiving team's 25-yard line if a kickoff or free kick after a safety results in a touchback.
- All fair catches result in the placement of the ball at the spot of the fair catch. Under NCAA rules (but not NFL rules), a kickoff or free kick after a safety that ends in a fair catch inside the receiving team's 25-yard line is treated as a touchback, with the ball spotted on the 25.
- Pass interference by the defense results in a 15-yard penalty, but no automatic first down.
- Pass interference by the offense results in a 15-yard penalty, from the previous spot, and no loss of down.
- The defense cannot return an extra-point attempt for a score.
- Any defensive player that encroaches the neutral zone, regardless of whether the ball was snapped or not, commits a "dead ball" foul for encroachment. 5-yard penalty from the previous spot.
- Prior to 2013, offensive pass interference resulted in a 15-yard penalty and a loss of down. The loss of down provision was deleted from the rules starting in 2013. In college and the NFL, offensive pass interference is only 10 yards.
- The use of overtime, and the type of overtime used, is up to the individual state association. The NFHS offers a suggested overtime procedure based on the Kansas Playoff, but does not make its provisions mandatory.
- Intentional grounding may be called even if the quarterback is outside the tackle box.
- The home team must wear dark-colored jerseys, and the visiting team must wear white jerseys. In the NFL, as well as conference games in the Southeastern Conference, the home team has choice of jersey color. Under general NCAA rules, the home team may wear white with approval of the visiting team.
- Since 2018, the so-called "pop-up kick"—a free kick technique sometimes used for onside kicks, in which the kicker drives the ball directly into the ground so that it bounces high in the air (thus eliminating the possibility of a fair catch)—has been banned. 
- Effective in 2019, NFHS gave its member associations the option to allow replay review in postseason games only.  Previously, it prohibited the use of replay review even if the venue had the facilities to support it. In Texas, the public-school sanctioning body, the University Interscholastic League, only allows replay review in state championship games, while the main body governing non-public schools, the Texas Association of Private and Parochial Schools, follows the pre-2019 NFHS practice of banning replay review.
At least one unique high school rule has been adopted by college football. In 1996, the overtime rules originally utilized by Kansas high school teams were adopted by the NCAA, although the NCAA has made three major modifications. Firstly, through the 2018 season, each possession started from the 25-yard line. Since 2019, this remains in force through the first four overtime procedures. Secondly, starting with the third overtime, teams must attempt a two-point conversion following a touchdown. Finally, Since 2019, the fifth and all subsequent overtimes are two-point conversion attempts instead of possessions from the 25-yard line, and successful attempts are scored as conversions instead of touchdowns.
Thirty-four states have a mercy rule that comes into play during one-sided games after a prescribed scoring margin is surpassed at halftime or any point thereafter. The type of mercy rule varies from state to state, with many using a "continuous clock" after the scoring margin is reached (wherein, except for specific situations, the clock keeps running on plays where the clock would normally stop), while other states end the game once the margin is reached or passed. For example, Texas uses a 45-point mercy rule (to stop the game) only in six-man football; for 11-man football there is no automatic stoppage but the coaches may mutually agree to use a continuous clock.
Robert Cantu, a Professor of Neurology and Neurosurgery and Co-Founder of the CTE Center at the Boston University School of Medicine, believes that children under 14 should not play tackle football.  Their brains are not fully developed, and myelin (nerve cell insulation) is at greater risk in shear when the brain is young. Myelination is completed at about 15 years of age. Children also have larger heads relative to their body size and weaker necks.  
Chronic traumatic encephalopathy (CTE) is caused by repeated brain trauma, such as concussions and blows to the head that do not produce concussions. It has been found in football players who had played for only a few years, including some who only played at the high school level.  
An NFL-funded study reported that high school football players suffered 11.2 concussions per 10,000 games or practices, nearly twice as many as college football players. 
According to 2017 study on brains of deceased gridiron football players, 99% of tested brains of NFL players, 88% of CFL players, 64% of semi-professional players, 91% of college football players, and 21% of high school football players had various stages of CTE. 
- "Concussions in High School Sports - Can Football be Saved? - Athletico".
- Bogage, Jacob (October 3, 2019).
"D-III football players say choice to forfeit season after injuries was theirs, not college's". The Washington Post. Retrieved October 3, 2019.
Nationally, high school football participation has declined 9.1 percent over the past 10 years.
- "2018–19 Football Manual" (PDF). University Interscholastic League. Retrieved January 21, 2019.
- "Section 159 – Football Rules". TAPPS Constitution. Texas Association of Private and Parochial Schools. Retrieved January 21, 2019.
- "Rule 69.1" (PDF). Rules and Regulations Governing Athletics. Massachusetts Interscholastic Athletic Association. July 1, 2011 – June 30, 2011. Archived from the original (PDF) on November 16, 2009. Retrieved July 28, 2009.
- "MIAA Aligns Rules with NFHS in Football, Volleyball & Baseball" (Press release). National Federation of State High School Associations. August 8, 2018. Retrieved January 21, 2019.
- "New Blocking, Kicking Rules Address Risk Minimization in High School Football" (Press release). National Federation of State High School Associations. April 24, 2018. Retrieved December 2, 2019.
- "Football Rules Changes - 2019". National Federation of State High School Associations. May 16, 2019. Retrieved November 24, 2019.
- "BCFOA Home". British Columbia Football Official's Association. Archived from the original on September 13, 2010. Retrieved September 1, 2010.
- Nader, Ralph; Reed, Kenneth (November 8, 2016). "The X's and O's of brain injury and youth football". Chicago Tribune. Archived from the original on August 27, 2017. Retrieved August 26, 2017.
- Cantu, " Concussions and Our Kids"
- Paul Solotaroff, "This Is Your Brain on Football" Archived October 1, 2017, at the Wayback Machine, January 31, 2013, Rolling Stone
- Toporek, Bryan. "New: High School Football Can Lead to Long-Term Brain Damage, Study Says". Education Week. Archived from the original on August 2, 2017. Retrieved August 26, 2017.
- "Deadly Hits: The Story of Ex-football Player Chris Coyne". Lauren Tarshis YouTube page. Lauren Tarshis. September 21, 2012. Archived from the original on May 28, 2017. Retrieved August 27, 2017.
- Preps at greater concussion risk Archived November 4, 2013, at the Wayback Machine, ESPN, Tom Farrey, October 31, 2013.
- "BU Researchers Find CTE in 99% of Former NFL Players Studied | The Brink | Boston University".
- "Comparison of Injuries in American Collegiate Football and Club Rugby: A Prospective Cohort Study - Nienke W. Willigenburg, James R. Borchers, Richard Quincy, Christopher C. Kaeding, Timothy E. Hewett, 2016".
- "The Common Types of Football Injuries".
- "Lower Back Injuries in National Collegiate Athletic Association Football Players: A 5-Season Epidemiological Study".
- ESPN College Football Encyclopedia by Michael McCambridge – lists all-time records for all current Division I and Ivy League colleges, including games played against high school teams ISBN 1-4013-3703-1