I started dating in middle school, though I didn't have my first real kiss until 9th grade. I also didn't drop out and I didn't experiment drugs until university. However, not all of my peers were so lucky, particularly those that started experimenting sexually at a younger age. Come to think of it, nearly all of the girls who I knew had their first sexual encounter by ninth grade (and were quite public about it) hadn't finished school/dropped out of college. I don't know what it's like for the guys.
I can't fathom the connection beyond the throw-away answer that they simply weren't ready for the psychological consequences (good and bad) of a physical/deeply involved romantic relationship at that age. Besides the complex emotions that were involved, there's social repercussions, particularly concerning society's attitudes towards sexuality.
Especially thirteen years ago.
These kids likely didn't have the proper emotional support; you can't talk about that sort of stuff openly with your peers, especially at that age. What kind of sounding board is a fellow eleven or twelve year old girl that is probably judging you for what you did? You can't talk to your parents about it either, for obvious reasons, and so really, it's only you and your not-quite adult brain tackling very adult emotional and psychological issues with the added, confusing burden of socially-imposed guilt and shame. On one hand, you're cool for rebelling against the 'system' and doing what you're not supposed to at that age, but on the other, you're shamed for it, especially if you're a young girl. Growing up teased and being called a slut for no reason other than preferring to play soccer with the boys in seventh and eighth grade instead of hanging out with catty girls, I can attest to how horrible middle schoolers can make you feel about yourself and your relationship with your sexuality. I can't imagine how it would feel if any of the things they were accusing you of had an element of truth. Those emotional scars would cut deep and probably disrupt healthy psychological development that may lead to drug abuse, depression, and alternative life choices.
I don't know about the soundness of the study, but I think it's a distinct possibility.