A teenage girl who appeared topless in a sexting cell phone picture that was distributed among her middle-school classmates should face child-pornography charges, a Pennsylvania prosecutor argued before a U.S. appellate court on Friday.

In the first U.S. case to test the constitutional status of sexting, the American Civil Liberties Union countered that the incident does not come close to meeting the definition of child pornography which typically depicts graphic sexual acts with minors and is done for commercial gain.

The ACLU also said the Wyoming County prosecutor erred when he threatened 16 teenagers with the felony charges unless they agreed to a participate in a re-education course on why sexting was wrong.

A three-judge panel of the U.S. Court of Appeals was considering a March 2009 court ruling that said the pictures, in which teenagers sent sexually suggestive pictures of themselves to their friends by cellphones or the Internet, fall under the U.S. Constitution's free speech protections.

Officials of the Tunkhannock School District in rural northeastern Pennsylvania contacted the county prosecutor's office in January 2008 after finding pictures on the cell phones of the 16 students.

Most of their families agreed to undertake a re-education program called for by the prosecutor, but three refused. Prosecutors were seeking to press charges against one of the three girls.

Pictures showed two of the girls wearing white bras and another standing topless outside a shower with a towel wrapped around her waist, the ACLU said. The pictures did not show any sexual activity.

MaryJo Miller said outside the appeals court on Friday that the picture of her daughter, who was wearing a bra, was originally taken in 2006 when she was 12 years old and attending a slumber party.

When she saw the picture, she thought the girls were goofballs, Miller told reporters. It was a training bra. You are going to see more provocative photos in a Victoria's Secret catalog.

But the county argued that the pictures were pornographic because they were disseminated for the purposes of sexual stimulation and so would be of great interest to child molesters.

Appellate Judge Thomas Ambro said prosecutors are not entitled to try to re-educate minors. I don't know of anything that allows the district attorney's office to play the role of teacher, he said.

The court is expected to rule within 90 days.

(Reporting by Jon Hurdle; Editing by Daniel Trotta and Vicki Allen)