There was a good recent article on this. I think it was in the New York Times, which means that since it has been more than seven days, it is no longer visible on the Internet for free.
Anyway, here is the cause. It is a cause that the CDC refuses to name because naming the cause is politically incorrect. In fact, it is so politically incorrect that even some here may not like it to be mentioned.
The cause is hysteria.
According to the article, when one schoolgirl (most rash "victims" are girls) gets it, so do their acquaintances. Doctors in the know call it "line of sight transmission." This does not mean that the girls are crazy. The author herself noted that just writing about rashes and itches causes authors to have, well, itches and a bit of a rash.
Although such rash clusters occur all the time, the event that seems to have marked the start of the current "rash" of outbreaks was not 9/11, but rather the anthrax attack.
This is not to say that all rash outbreaks are psychologically caused. Sometimes an entire cluster has a medical or environmental cause. However, this a quite rare. Sometimes one person will break out from an underlying medical cause and others will break out in sympathy. Breaking out in sympathy is not a bad thing. Some of my readers are probably itching right now, and there may be a small rash at the itch site. If anyone has a really big rash now, my post has succeeded far more than I had hoped was possible :-)
There are no "good" articles in the New York Times.