The Federal Department of Criminal Investigation Department's heavy and organized crime department seized extensive evidence after they had searched 21 homes in 11 federal states at the end of October: cell phones, computers and tablets. Their owners, however, were not gang criminals with criminal records. They were teenagers, normal teens, the youngest 14 years old.

The suspects did not know each other and yet are all accused of the same act. They'd sent videos around on their cell phones, shared them on social networks, posted them in chats. That they have not thought anything bad, you can see from the fact that they provided their postings with all sorts of LOLs and laughing emojis. But the videos were not harmless. It was child pornography.

These films were not distributed in the back rooms of the Internet, in the secret, password-protected pedophile forums or in the Darknet. But where children and teenagers prefer to stay when they are online: at WhatsApp, in the group chat of the school class.

Founded in 2009, WhatsApp instant messaging service is a part of life for teenagers today. As natural as chic sneakers, homework or stress with parents. Studies estimate the proportion of young people who regularly use WhatsApp at 97 to 99 percent. If you ask them which online offer they would least miss, they call WhatsApp by far the most common.

In addition to speaking, WhatsApp, which has been on Facebook for five years, has become the most important communication tool for young people. When they make an appointment, they do not call each other, they write. Or they write instead of dating. When they visit, they do not ring the bell, but use WhatsApp to say, "I'm getting down, open the door." Typed texts and emojis fly from screen to screen as well as photos, videos, stickers and voice messages. Jokes, declarations of love, bedtime kisses, insignificant and profound things.

Up to 256 participants can join together to form a WhatsApp group, in which everyone has equal rights. In this way, teens organize parties and festivals, shopping, skating and Fridays for Future demos. No theater group, no football team, no youth fire department, no big band, no scout tribe without WhatsApp group. And no school class. In Germany, there is practically no class today without an associated chat.

Behind the nationwide, nationwide spread is no plan. Class chats are not offered by the schools, but always unofficially. The creation is done by some pupil, little by little everybody joins in - let's go.

And then go the problems.

"There is probably no school in Germany where disturbing content is not an issue at WhatsApp," says Heinz-Peter Meidinger, director of a Bavarian grammar school and chairman of the German Teachers Association. Most of what is posted in class chats, although unproblematic: There is talk about clothes and music, chattered and laughed, "normal communication with young people halt". But again and again it is "problematic, sometimes highly problematic," says Meidinger.

In February, the public prosecutor's office at Tegernsee Gymnasium investigated the "upper grades" for "objectionable images in a group chat".

In March, it was revealed that a boy had posted porn in 11th-grade chats at a high school in Essen - the director called the police.

In April, the police confiscated the phones of all 24 students in a 9th grade class at a Stuttgart school after child pornography files had been sent in class.