I think that, sadly, Mark most likely committed suicide.
To find out that your girlfriend is with your best friend is problematic at any age, but way more so at age 15 when emotions and hormones are running wild, especially if this was his first time in love. He reportedly left in tears. I can see this double betrayal making him more and more despondent now that he was out of the house and alone with his thoughts in the dark of the night.
Alternatively, he could've decided to not get on the bus but rather go for a walk first to clear his head, but in his highly emotional state he didn't notice he was walking into the path of an opportunistic predator.