Can a student trespass on school property during a public event?

Hooray for the Lewisville School District for dropping the criminal charges against two Flower Mound High School students who pulled a prank during a football game last week. I don't understand, however, why they were even arrested in the first place. Why wasn't discipline handled exclusively by school personnel? And what gets me is the quote from one Karen Permetti, identified only as a "spokeswoman for the school district," who said the students should have been charged with "trespassing on school property." Huh? How could they be singled out for trespassing on school property" and not the football players, the bands, the drill teams and every single person, especially those non-students, who attended the game?