BOSTON -- Parents are being warned Thursday about a case of sexting at a Sudbury middle school.

The superintendent sent a letter home to parents after police notified the district about the incident, which involved Curtis Middle School students sending inappropriate pictures by cell phone and the Web.

Sudbury police offered few details about the case, but said the ongoing investigation involves middle school students sending nude and partially nude pictures via the Internet and cell phones. The incidents did not occur on school grounds and no charges have been filed, police said.

Superintendent John Brackett said he wants parents to get involved at home and for the school system to reinforce safety lessons. He also talked about the role media, movies and video games have in society.

"If that's the medium that's defining what's right and common or OK, then we have to strike back and help young boys and young girls have more self-respect for themselves," he said.

The Sudbury incident comes a week after another case of sexting in Belmont.

Police said an eighth-grade girl at Chenery Middle School took a nude photo of herself and texted it to her eighth-grade boyfriend.

The boy then allegedly sold the picture to friends for $5 apiece. Police have confiscated cell phones from several students.

Child pornography charges in the Belmont case are possible.