It went far beyond annoying, and with the apparent approval of an assistant principal.
So, what would you charge him with. Involuntary manslaughter might be appropriate. Never going to get first or second degree murder in this situation.
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.