If he brought the gun to school that day, then it looks like premeditated murder. It sounded like he waited till the back is turned too. Now whether this should be in juvy or adult court is another matter, but it looks like 1st degree murder to me if I was a prosecutor.
If they were adults, I'd agree with you. They aren't, so I disagree.
Did you know that in many states a 12 year old child is presumed to not be able to form criminal intent. In the state of Washington, if an eight year old child shot and killed someone, they could not be convicted of a murder even if the child intented to cause that death?