I suspect it is disinformation (either by the authorities or the sniper) however, since nothing was left at the other scenes.
I also suspect the sniper may have a connection of some kind to that particular school. I don't think that it's a coincidence that the child he shot is thirteen. Thirteen is the age of accountability in Christianity and the age of bar mitzvah in Judaism. Since muslims have "borrowed" from Judaism and Christianity to arrive at their warped beliefs, I suspect they have a similar belief--which means the sniper could justify shooting this child on the grounds that he really wasn't a child anymore.
I also suspect that we are looking for a black muslim from an al Fuqra cell. It's entirely possible that someone in his family knows this child at least enough in passing to know that he is thirteen and that he would be arriving at school at a time when a clean shot was likely.