Having sex with a 15 year old girl is against the law, but, yes things are not as black and white if the guys are under 18 themselves, but it still could be prosecuted. Even if she's a willing participant. Typical though to blame the victim - if she didn't want to be bullied she shouldn't have had sex?
The main point here though is the bullying. It's wrong and when it leads to a suicide people need to be held 'accountable' - personally accountable just like you said.
When the victim kills themselves no one cares. When the victim decides to turn things into a Columbine then oh know the person who was being bullied is sick, crazy, demented etc. Lets blame the guns or the gun dealers. Lets not look at the root cause which was the bullying.
NY needs to pass anti-bullying laws just like Mass did.
http://www.slate.com/articles/life/bulle/2010/04/bullies_beware.html