THE FIRST PUNCH OF THE RUMBLE WAS NOT FAKE.
During the rumble scene, and a little after the line where Matt Dillon (Dallas Winston) says, "You know a rumble ain't a rumble without me," and C. Thomas Howell (Ponyboy Curtis) gets the first punch of the entire fight, Tommy later claimed that the punch was most certainly real and knocked him out for about an hour and a half before they started filming again. He says that it was really painful, and quite frankly, the entire filming crew found it hilarious, hence having it lead up to the director deciding to keep it in the movie.