My Son Is Sending Nude Pics Of His Sister

I got a phone call from the father of one of my son's friends. He told me to go check my son's phone. Turns out my son and his friends have been sending inappropriate stuff to each other. They have been sending each other pictures, not of themselves but of their sisters. I scrolled through his pics and discovered he had sent quite a few pictures of his sister. My daughter sleeps naked but she also has her own room. He snuck into her room removed her sheets and took pictures of her. I got even angrier when I read some of the messages. His friends were doing the same taking pictures of their sisters sleeping or in the shower. One kid texted in that he wanted to fuck my daughter and sent a video of him jacking off and cumming on the picture of her. Now I'm stuck putting a lock on my daughter's door, which I know I am going to regret at some point, along with figuring out what to do with my son and his friends. I am quite sure the other parents aren't going to do anything. I've been looking at our finances to see if we can afford military school.