So, lemme get this straight: The doctors can't fix Trevor's stab wound? Maybe I'm just underestimating his injury, but it really seemed like something that could be treated, especially since everyone acted immediately about it.
A person can quickly die from internal bleeding from abdominal trauma. Besides all of the "solid" - blood rich - organs like the liver, kidneys, and pancreas, the blood is pumped out of the heart through a huge artery called the Aorta which immediately splits into two arteries; the Ascending Aorta which goes to the upper body and head, and the Descending Aorta which goes through the chest and abdomen bringing high-pressure, oxygen-rich blood to the lower body. If the Abdominal Aorta was punctured from a stab wound, the high-pressure blood would be forced into the abdominal cavity with such force that the person would bleed to death internally within a few minutes.
John Ritter was killed by a condition called an Abdominal Aortic Aneurysm (AAA) that results in a spontaneous rupture of the Abdominal Aorta without any outside trauma at all. It just happens.
It is completely believable, and likely, that a 12 y/o boy could die from an abdominal stab wound.
Wait, so shouldn't the bullies get charged for, you know, murdering someone?
I'm sure they did, they just didn't show it. The point of the ending isn't to show the bullies punished, it's to show that Trevor changed the world after all.