I'm looking for a short story I read recently, probably in a sci-fi anthology. It is about a group of schoolchildren who form a secret club in a disused classroom, and membership in this club is dependant on their ability to look at a picture on a piece of paper for more than 10 seconds. They hold competitions to find out who can stand to look at it the longest.
The image in question is a kind of computer virus for the human brain, entering through the optic nerve. Terrorists are using a different version of this image to kill people. The one the children have causes intense discomfort physically and psychologically.
The children think that certain areas of their school are in permanent darkness, but that is in fact their perception being altered by a surgical implant to protect them from these images in uncontrolled areas.
If this sounds familiar to anyone I'd love to know the author or what it was called! I hope I've remembered the details correctly, don't want to write the ending because of spoilers!