What happened at the beginning?

  Well, I've read some part of the book and now I can tell what happened there.
It was usual day in the Garvin High School. Everything was like the any other day. Students were gathering in school cafeteria for lunch. The shooting began very suddenly. It was real tragedy. Six students died and others were wounded. A certain Valerie Leftman shield other girl by herself or just fell on the shooter, but she was the last victim before Nick Levil, the shooter, turned the gun on himself. Then I learned that Valerie was a girlfriend of Nick. Hmm, it's strange. Everything is strange. Why had he shot? And why had he killed himself? Had Valerie known what he was going to do in that morning? Why had she jumped between Nick and other girl, Jessica?
  Valerie Leftman have kept her bed for one or two week yet, because her thigh wounded. But her mom insists on going back to school, because Valerie's doctor says that she can do it. However, Valerie can't go back. She is scared of facing all those people. Some of them think that she is a hero, others think she is a murderer. But actually, she doesn't know who she is. Some days she felt like both, some days she felt like neither. And she didn't try to save a life to Jessica when she jumped between her and Nick. This girl pestered Valerie.
  I think that her mom sooner or later will make her go to school. What will happen next? I have a lot of questions. I'll write answers down as soon as I'll know.