Er. Ow. And possibly "ack!"
I kind of got blind-sided by this one, at least at first. The description of the boy in the plum tree yelling philosophy at his classmates has me expecting something a bit more serio-comic. But the situation gets dark very quickly, and even before things get really horrible it's easy to see that's where it's going.
Although the characters are early teens, I recommend giving it a read through before handing it over to any sensitive thirteen year olds, or sensitive souls of any age.