The word is "sexting" and the process it refers to is the practice of sending sex related texts and/or images. In a survey referred to by a US website ("Houston schools ban 'sexting'") it's claimed that 22% of teen girls have sexted- in other words, they've sent or posted nude pics.
Education authorities are concerned what might happen if students text their boyfriends/girlfriends explicit images and they fall into the wrong hands. And of course another big concern are child pornography issues related to images of underage children.
The article quotes Hans Graff, assistant general counsel at the Houston Independent School district, stating that: "Some principals raised the issue, so we included a provision saying sending, receiving, possessing sexually suggestive messages is forbidden" ("Houston schools ban 'sexting'").
The proliferation of this kind of sexual activity is obviously a problem. But how do you explain the dangers to impressionable teenagers when their celebrity role models are doing exactly what they are being told not to?