WAUKESHA, Wis. – Police contend dozens of Waukesha high school and junior high students are trading nude images of female students “like baseball cards” and several are expected to face criminal charges.

Four to five male students set up a group discussion on the smart phone app Kik to share nude photographs of female students throughout the district, and other students took screen shots or saved the pictures and shared them on other social media, Waukesha police told Fox 6.

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A parent sparked the investigation when she found a naked image of a girl on her son’s phone and contacted the Waukesha Police Department, officials said.

“We looked into it and discovered that there were four or five male students who created a repository of photographs categorized photos using Kik social media,” Waukesha Sgt. Jerry Habanek told the news site.

Many students saved the images from the Kik group to their own computers or phones hidden behind special apps that appear to be something they’re not, such as a calculator.

“Kids can buy it, install it and basically it’s a secret compartment for them to hide things, pornographic pictures – things they don’t want you to see,” said Brian Dorow, Dean of Criminal Justice and Homeland Security Waukesha County Technical College.

Students would then trade the images with each other using other social media apps including Snapchat, Facebook messenger, and I-message.

“What these students were doing or other students were doing, is screenshot the photos or they’d use a third party app to save the Snapchat photo. They’d save it and contact one of these five boys who had the repository and, say I have a photo of ‘Suzy,’ I’d like to trade it for a photo of ‘Nancy.’ Just like trading baseball cards, these guys would trade photos,” Habanek said.

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Waukesha School District Assistant Superintendent Joe Koch told WISN the images were not taken on school property or during school time. Police said the images involved girls between the ages of 10 and 17 at Waukesha North, South and West high schools, as well as Butler and Horning middle schools.

But it wasn’t just pictures students were sharing.

“The other thing they were trading was not only photos but short videos of sex acts with themselves,” Habanek told Fox 6. “Those types of things were being sent, shared and now traded.”

Waukesha police believe between 25 and 30 students participated in scheme, and plan to request child pornography charges from the district attorney’s office about two dozen students, according to the news site.

School and police officials are now encouraging parents to comb through their children’s cell phones to determine if the devices contain child pornography.

“I am going to check my son’s phone … as soon as he does get home to make sure he’s not a part of it,” one mother told WISN, “I hope he’s not.”

The investigation isn’t the first time Waukesha police have investigated local students for taking and sharing nude images online. In September, investigators came across nude photos taken by Horning Middle School students on school-issued iPads,

“Initially, before I began doing what I’m doing, I was surprised,” Habanek told TMJ 4 at the time. “But now it’s so commonplace, and I think it’s commonplace because middle school kids are using middle school minds and doing adult things.”