The Columbine happened in April 1999 and dominated the news cycle for a long time. It likely had a big impact on a generation of American kids. Kids that would otherwise have proceeded straight to a fistfight may have had second thoughts in case the person attacked showed up the next day with a rifle.
As for later years, schools took a zero-tolerance policy, so that would have cut down on it significantly. With social media use increasing towards the end of the 2000s, fights could have been reduced further because people did not want to end up as a punchline (no pun intended) on Worldstar or Vine.
This is probably the answer right here. Fighting now means you'll have video fight evidence gathered from multiple angles, backed up to the cloud, and then potentially used as a societal-attack on you and your family, potentially for years or into perpetuity.
As for later years, schools took a zero-tolerance policy, so that would have cut down on it significantly. With social media use increasing towards the end of the 2000s, fights could have been reduced further because people did not want to end up as a punchline (no pun intended) on Worldstar or Vine.