One summer night in 2007, a pair of 13-year-old northeastern Pennsylvania girls decided to strip down to their skivvies to beat the heat. As Marissa Miller talked on the phone and Grace Kelly flashed a peace sign, a third girl took a candid shot of the teens in their white bras.

It was harmless, innocent fun, the teens say.

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But the picture somehow wound up on classmates' cell phones, and a prosecutor has threatened to charge Miller and Kelly with child pornography or open lewdness unless they participate in a five-week after-school program followed by probation.

On Wednesday, the American Civil Liberties Union asked a federal judge to block Wyoming County District Attorney George Skumanick Jr. from filing charges, saying that the teens didn't consent to having the picture distributed and that the image is not pornography, in any event.

The photos surfaced in October, when officials at Tunkhannock Area High School confiscated five cell phones and found that boys had been trading photos of scantily clad, semi-nude or nude teenage girls. The students with the cell phones ranged in age from 11 to 17.

Called "sexting" when it's done by cell phone, teenagers' habit of sending sexually suggestive photos of themselves and others to one another is a nationwide problem that has confounded parents, school administrators and law enforcement officials.

"There was absolutely nothing wrong with that photograph," said Marissa's mother, MaryJo Miller, 44, of Tunkhannock.

"I certainly don't want paedophiles looking at my daughter in her bra, but I don't think that was the intention to begin with," she said. "This is absolutely wrong on his part. It's abuse of his authority."

A hearing on the ACLU's request for a temporary restraining order is scheduled for Thursday in Scranton.

Marissa Miller, Kelly and their parents were joined in the lawsuit by a 17-year-old Tunkhannock girl from the same school who was photographed topless last year in a separate setting. The teen, also facing felony charges unless she takes the class, is called "Nancy Doe" in the lawsuit to protect her privacy.

Skumanick said he would fight the lawsuit.

"Frankly, we just wanted to protect these kids, and say, 'Doing this is not right.' We wanted to offer this course to educate them, and make them understand the long-term ramifications of having photos like this out there," he said.

Prosecutors in a number of states, including Pennsylvania, Connecticut, North Dakota, Ohio, Utah, Vermont, Virginia and Wisconsin, have tried to put a stop to it by charging teens who send and receive the pictures.

In Tunkhannock, a town about 210 kilometres north of Philadelphia, Skumanick met with about 20 students and their parents last month and offered them a deal: The youths wouldn't be prosecuted if they took a class on sexual harassment, sexual violence and gender roles.

"If we were just out to be nasty about this, we would have just charged everybody," Skumanick said.

Seventeen of the students accepted his offer.

But when MaryJo Miller protested to Skumanick that the photo portraying her daughter and Kelly couldn't possibly be considered child porn, he replied that the girls were posed "provocatively," according to the lawsuit.

Marissa Miller, now 15 and a freshman, said nothing could be further from the truth.

"It was a first-ever bra that your parents buy you, an old grandma bra," she said.

"I think it was even a padded bra," her mother added.

But Skumanick said the image wasn't appropriate.

"You'd have to see the photo and see the setting, which I'm not going to show to the public because I don't think it's appropriate," he said.

The year-old photo of Nancy Doe, meanwhile, shows her just out of the shower, with a towel wrapped around her waist and her breasts exposed.

Vic Walczak, legal director of the Pennsylvania ACLU, would not say who took the photo or whether she sent it to anybody. But he said she did not intend for it to be widely distributed.

"Our client has absolutely no idea how it ended up on strange people's cell phones," he said.

Under Pennsylvania's child pornography law, it's a felony to possess or disseminate photos of a minor engaged in sexual activity, "lewd exhibition of the genitals," or nudity that is meant to titillate. Open lewdness, a misdemeanour, includes any lewd act that is likely to be observed by others.

The ACLU's lawsuit claims both photos are protected First Amendment speech.

"One of the concerns all of these kids have is it's getting to be summertime, they're going to the beach and the pool, and do they need to be worried about taking photos of themselves swimming around?" Walczak said. "Who knows what he decides will be too provocative?"

Skumanick said swimsuit-clad kids have nothing to fear from him.