In the first federal appeals court opinion dealing with “sexting” — the transmission of sexually explicit photographs by cellphone — a three-judge panel of the United States Court of Appeals for the Third Circuit ruled Wednesday that parents could block the prosecution of their children on child pornography charges for appearing in photographs found on some classmates’ cellphones.

“It does not resolve all of the constitutional issues implicated in sexting prosecutions, but it’s a terrific start for civil liberties,” said Witold Walczak, legal director of the American Civil Liberties Union of Pennsylvania, who represented the parents.

The case, Miller v. Mitchell, began in 2008 when school officials in Tunkhannock, Pa., discovered seminude and nude photographs of some female students — some as young as 12 or 13 when the photographs were taken — on other students’ cellphones. The officials confiscated the phones and turned them over to the Wyoming County District Attorney’s Office.

The district attorney at the time, George Skumanick Jr., said that students possessing “inappropriate images of minors” could be prosecuted for possession or distribution of child pornography, and sent letters to the parents of the students with the phones — and the parents of students who appeared in the photographs — threatening to prosecute any student who did not participate in an after-school “education program.”