If your son did not invite the pictures, and he showed them to you as a means to get advice on what to do, then your son is not guilty of "sexting"...which is a recent law used to address kids doing just this type of activity.  However, if you do go to the school or law enforcement, then you run the chance of the girls saying "oh...they told us on the phone to send them the pictures and that's why we did it..."---because they want to reduce the pressure on their liability.  With this in mind, I suggest you delete the photos and block the girls from sending your son additional photos.  Your son needs to understand that possessing these types of photos can result in him being placed on juvenile probation, being sent to an alternative school campus, and potentially being sent to a juvenile detention center (kiddie jail).  It's stupid.  I know a good number of young kids are doing it...but the cops and school officials are not consistent in who they decide to make an example of.  Meaning, they may not through the book at the popular kid, but they will still go after the kids that have not been in trouble.If you really want to tell the school about the pictures, I suggest that you hire an attorney to go with you to report the events.  You will need a criminal defense attorney.  Sometimes when you have that little extra muscle, they are not as inclined to make an example of your child...and instead focus on the young ladies that took and sent the pictures.