I boarded 13-18
My school was known for being small, nurturing and progressive - and it was, to an extent.
The bullying for the first two years was so bad it drove me to self-harm and suicidal ideation - and I certainly wasn't the only one.
The school advertised itself - as all boarding schools do - as having a full programme of enrichment at the weekends, great facilities and clubs and activities, yada yada - it did have great facilities, if you were a horsy person or a boy who liked squash/cricket, but most of us spent the weekends hanging around in a daze watching tv and eating supernoodles. There were long, long afternoons of absolutely fuck all going on.
I've worked in schools (state schools, mostly) and never have I seen the level of drinking, drug-taking, bullying, underage sex, sexual assault, bullying and general dangerous behaviour. Supervision in these places during unstructured times is MINIMAL, whatever they tell you.
The worst aspect of it, for me and many of my peers, is the emotional deprivation; I recently came across the term "skin hunger" while researching something else, and I sat and cried. It described exactly how I felt during my boarding school years. You spend weeks at a time not encountering anyone who loves you. No hugs. No arm around you when you're distraught. However friendly and professional and wonderful a teacher might be, they don't love you, and they're not allowed to touch you. I think I smothered my kids a bit by way of compensation.
I'm not saying there aren't positives to boarding; of course there are, or nobody would choose it. Our school had a farm, and fabulous art/music/sports facilities. There were frequent trips to see the RSC in Stratford, the teaching was good (though not, interestingly, better than the decent state schools I've known). We did make some lifelong friends; they're more like siblings or cousins, really. I wouldn't send any child in my care to board though, because the downsides hugely outweigh the positives and the scars just aren't worth it. And it's often the ones who will say they "loved it" who are the most scarred, emotionally.