So how did the massacre in this movie work?

He locks some of the doors to the gym, easily visible from the outside. Then he goes inside the gym and assembles the bow and waits for a school class to come in through the only unlocked door. This school class shouldn't notice him in the best case.

Then he starts shooting arrows at moving targets (very difficult). And none of the people rush at him to stop him. They also don't leave through the remaining unlocked door. They don't beat him up either when he runs out of arrows, apparently he's so lethal with a bow he didn't leave a single survivor.

... but actually even the two arrows that supposedly killed his father didn't look too fatal. One got stuck in the lungs, alright, that sucks, but the other one was merely in his arm. He was still lying just dead on the lawn lol.

I would say realistically in an average gym, he could fire about 3 arrows before someone gets to him and beats him to the ground, assuming he has some place to hide.

But he can't hide AND lock the remaining door. So that problem is just unsolveable.