Updated 02/12/13 – 10:19 a.m.

MUNSTER, Ind. (CBS) – A dozen students from a northwest Indiana high school have been suspended for three days, after sending partially nude pictures to each other.

Munster School Supt. Richard Sopko said officials found photos of two teenage girls – 15 and 16 – on students’ phones at Munster High School.

He said the photos were taken by the girls’ ex-boyfriends last summer, and were sent by phone to other students at the school.

A girl who thought her picture was being shared with classmates reported the incident to her mother, then the school.

It turned out that girl was not in the photos, but two other girls were.

Sopko said the cell phones belonging to 30 students were checked, and 12 had the partially nude pictures. The texting that led to the suspensions began on Thursday.

“These are good kids. They just made a bad mistake,” he said. “The whole image sounded like it was this mass plot, and that these are terrible kids. That really is not the case.”

The three-day suspension started on Friday.

Police were also investigating.