A federal appeals court on Friday takes up the growing practice of “sexting” — in which teenagers transmit nude and semi-nude photos of themselves and others by phone — as the judges tackle the vexing question of whether such images can be deemed child pornography.

The appeal in Miller v. Skumanick stems from a civil rights suit brought by three Wyoming County, Penn., girls against then-District Attorney George Skumanick Jr. alleging that he violated their First Amendment rights with his threat of a child pornography prosecution if they refused to take a class he had designed to educate youths about the dangers of sexting.