Reversing a judgment that tossed a high school student’s First Amendment claim against a principal who barred him from wearing a T-shirt with a picture of a handgun, the 7th U.S. Circuit Court of Appeals explained that the district judge made a mistake — “but perhaps an understandable one” — in failing to apply Tinker v. Des Moines Independent Community School District, 393 U.S. 503 (1969), the trailblazing case about students who wore black armbands to protest the Vietnam War. The gun depicted on A.L.’s T-shirt was part …