Kids do spend more of their time at school than at home during the school year - more of the time they are awake that is. However, while at school, high school and jr high teachers are seeing a particular student for only an hour to an hour and a half each - along with another 20 or so students. And, during that time they are supposed to teach the entire class a lesson. While they are trained to spot concerning behavior (as 2 seperate teachers did in this case) on a regular basis, they would not have time to sit down and check in with every student during class time.
Parents, on the other hand, theoretically should have that time and be checking in regularly with their kids. The reports said these 2 instances were the first time Ethan had interacted with the Principle's office, so truly none of them (Parents' or school) probably could have imagined what he had planned - BUT - the school would not have known he had access to a gun. Obviously the parents did. If they showed no concern, did not divulge he had been given and had been handeling a gun recently, and refused to take him out of school, I would think the school officials had no choice but to keep him the rest of the day. They would probably believe he would be safer because frankly, what he wrote at least on that drawing, sounded more like he was thinking about self harm than harming others. IMO.