Mistakenly believing that a middle-school classmate had caused the arrest of a friend for bringing a gun to school, three 14-year-olds and a 13-year-old in Fort Myers, Fla., posted Facebook messages discussing the killing of the suspected “snitch.” The students — three of them girls — were arrested at school and charged with felony aggravated stalking. The threats against the unidentified boy victim were discovered by his mother, who notified sheriff’s deputies.