That has to be so heartbreaking, now and even then when you just "knew." A teacher, no matter how good, how compassionate can't fix everyone. You got this boy 5 hours a day. The other 19, he had whatever he had at home. God only knows what he endured, what made him unable to trust.
I was never a teacher, but I was sort of the unofficial mom to all my sons' friends. My home was the safe place and I was the mom they could come to. Some can't be saved. Small story, a gf my youngest son had. She confided in me that she was planning on ending her life. She was a minor. I told her I'd have to tell her parents. I did. They did NOTHING. I repeat, NOTHING. Rather than be grateful someone cared about their daughter, they acted like I was an intruder. She continued to engage in self-destructive behaviour and has been in and out of trouble and on various drugs for ages now.
The story about the kids in the car is horrifying, my heart breaks for everyone involved.