It may have been years since I researched this topic, and it may have been in a Pennsylvania public school that the paper was written, but here's what I can recall from memory about the 4th Amendment to the US Constitution, specifically about illegal search and seizure and how it relates to public schools:
Police entering the school to complete a search are just that: Police. As such, they are bound to the full effect of any Local, State, and Federal laws regarding search and seizure. That part is clear-cut and dry. Immediately after that, however, it gets fuzzy.
For example, a student's locker is their personal space, right? Not always. It's government property, but there is a confidence held with the school that possessions stored within specifically designated areas will remain private. This has gone both ways in court, and largely depends on the circumstances.
If the police want to avoid that whole argument, then they have the easiest of ways to have that space searched and items collected: school administrators. This is where a student will realize that, because they are under 18 (and under 21 in some states), they have very little say in the situation.
Police need Probable Cause to search without a warrant. School administrators need only "Reasonable Belief", also called "Reason to Suspect" or one of many other phrases. As long as the student or the property are on school grounds, a school administrator has full and complete privilege to any of that students belongings, and the option to detain the student against their will until Police arrive.
So, what constitutes Reasonable Belief? Quote simple, really: anything at all. Did the kid look funny? Did the administrator think they overheard a foul comment? Reason to believe.
This may have been a long way of getting around to it, however the fact remains that this cell phone was taken in accordance with the law and is fully permissible as evidence. It doesn't matter why the administrators were looking through the kid's pictures, they can claim anything now.
The real test of law here is whether child pornography prosecution can be used against minors who willingly took and distributed the pictures of themselves. Furthermore, can the boys be charged for receipt of something they did not have the option to reject? I don't know about you, but I don't have a choice to reject an SMS on my phone, it just accepts it no matter what.