I cannot fathom how young, you are as an author. At seventeen, I wasn't half the writer you are, and I know if you keep perfecting your craft you will be successful.
Read the story now
Your characters need a little more depth. This is easier achieved by writing your point of view from a limited third person than an omniscient third. In a limited third or a first POV, you find the raw emotions, the conjecture, the motivations. This does not have to be just Dylan's side of the story. You could likewise do Jack's or David's or Ann's in that limited 3rd per chapter. I challenge you to read each chapter. Who is that chapter about? Who do you want to tell it? Write it. You don't have to keep it. You could stay with the Omniscient 3rd and use the limited as part of the writing exercise. What is great about this exercise is that your characters will speak. They will tell you their stories.
This book's plot is great, but in drama, there must be hope. Where is Dylan's hope? Or David's? What is going to keep them climbing out of bed in the morning? Resolution is great but by the end, we want to see wounds mending not blood staining the bandages. I'm not asking for a happy ending. I'm asking for your characters to have a genuine closure that your reader can relate to as well. The final scene--total set up for a sequel. Well done!
Overall, I'm so impressed by your work. Please keep writing! You have a true gift! Remember, writing is 90% Revision!