Entirely possible, and plausible to me...
17 yr old has gun at school. 1st wrong thing. Gun is loaded. 2nd wrong thing. Loaded gun in backpack. 3rd Wrong thing. Gun fires when backpack is dropped on desk. 4th wrong thing. Student ditches backpack. 5th wrong thing. Surrenders to police. 1st right thing.
and they don't "go off" when you toss the container they're in onto a desk.
It might if the gun is loose in the backpack (along with other stuff) and is one of the models that have a "safety" on the trigger. It is entirely plausible that something else in the backpack could have pulled the trigger when the bag was dropped/tossed on the desk.
I read three of the reports on the shooting, and if the facts are as claimed, that the gun went off when the bag was dropped, it would be an accidental discharge, (actually negligent) in the sense that the kid did not intend to shoot, or aim at any one. Yes, he is certainly responsible, but the shooting was apparently not a deliberate act.
A gun like a GLock, not in a holster or case, loose in a backpack with books, pencils, pens, and whatever else a teenage boy might stuff in there, with a round in the chamber, could certainly be discharged in the manner described.
The news reports conflict in some degree, one report saying he took the gun out of the pack and pointed it, one saying the bag was dropped on the floor, another says on a desk. Three reported injured, reduced to two struck by the same bullet, one in the neck, the other in the head. Also saying the kid ditched the backpack (apparently with friends help, as it is reported two more kids were arrested), and police have not recovered the backpack. They did recover the gun.
It matters not to me why the 17yr old had the gun, he is responsible for the shooting. The fact that the gun went off in an unplanned manner does not absolve him at all.
And before we get all set to burn the kid and the parents, we need to know if the parents were actually negligent in letting him access the gun. Right now, nobody is saying where the gun came from. Even if it did belong to his parents, if he stole it from them, I don't see them as responsible. Parents can teach, and take every possible precaution, and still have their children deliberately do wrong things. The parents may be at fault. THey may not be, we just don't know that...yet.