Continuity mistake: When Alex walks forward in the cafeteria with his notepad, he stops by the cafeteria tills. In one shot, you see a student going up to pay for their lunch. But it then cuts, and the student has vanished.
Continuity mistake: Early in the movie, just before John is seen leaving the school and passing the gun-toting students for the first time, John is shown briefly coaxing a dog into a little high-jumping trick. Much later, when the same scene is viewed from the gun-toters perspective, the dog doesn't do the trick.
Continuity mistake: Due to the different perspective during the whole day, there are some inconsistencies in the events during the day, for example when John leaves school early on in the movie it is a dry day. But later on, when shown from a different perspective it is very wet as he leaves.
Continuity mistake: When Michelle goes to library and places the books on a bookshelf, the top three shelves are full of books and the bottom shelf is empty. But after this scene, all the shelves are full of books, including the bottom one. (00:37:15 - 00:57:50)
Answer: This movie was based on the Columbine High School Massacre, and another high school massacre. There's no answer to "why" they did it.