Thanks for all your opinions and suggestions - I'm glad the majority wouldn't find the characters irredemable. I didn't plan for the victim to die though - he is unconscious and in their panic and the near-darkness, with the blood, they think he is. But he is found and survives, and doesn't know who hit him. They think that keeping quiet is best for everyone - although they are struggling with guilt, they think it will do harm to too many people if they come forward. But there's a witness, who the mc must track down and persuade not to tell (and which becomes a large part of the conflict in the book).
And to those who mentioned I KNOW WHAT YOU DID LAST SUMMER - I'm aware of the parallels, although I've had different versions of the hit and run idea in my head for the last 5 years (before I had heard of IKWYDLS) I hope it won't matter too much how close the inciting incident is to the other book ( my version wouLd take it in quite a different direction I think?)
Just in terms of inciting incident parallels (and since the victim lives), this also brought to mind BONFIRE OF THE VANITIES for me. That's obviously not remotely YA though, and I'd guess your story would go in a completely different direction.