Facing those choices on a large scale now, in the glare of national attention, is Thom LeDoux, the district attorney in the county that includes Cañon City, Colo., where more than 100 students at the high school were apparently exchanging nude picture s.

Mr. LeDoux said in a telephone interview that he does not plan to file charges against those who simply passed around pictures. But felony charges could be in store for some, he added — if an adult was involved; if there was evidence of coercion, illegal sexual activity or bullying; or if pictures were posted on public websites.

Erotically charged cellphone pictures or videos passed around by teenagers often meet the legal definition of child pornography, making them the subject of felony laws that were written with true predators in mind. So when a 16-year-old girl emails a raunchy picture of herself to a boy, she has in theory created and distributed child pornography. If the boy sends the picture to 20 others, he has distributed, and they all have possessed, child pornography.

Yet few prosecutors want to ruin the lives of teenagers for one-time displays of immaturity. Across the country, district attorneys are forced to decide: When are youthful actions so malicious or harmful that they should be prosecuted? When are serious felony charges and perhaps lifetime branding as a sex offender called for, and when is probation and, perhaps, community service or counseling enough?

About 20 states have adopted new laws intended to address juvenile sexting by providing a less severe range of legal responses to personal photo-sharing, including misdemeanor charges that may be expunged, and required community service or counseling. But even with these more subtle tools, the prosecutors have to make delicate calls.