My DDs went to a very selective girls' school. In the my older DDs year there really were none of these issues. I might be able to say that a couple of my DD's friend's self esteem and confidence may have suffered from not getting the best results in the year. B really is for bad from the perspective they gain in these schools, completely out of perspective by real life standards. However nothing that you could define as a mental health problem.
DD2s year was very very sad. Several girls did end up having not just counselling but more intensive intervention and residential help with depression, eating disorders and drug and drink dependency, from a scarily young age. It may have been the school wasn't the right environment for some of the individuals but the problems definitely originated in the home environment and then spread into affecting the whole year. There was a lot of attentions seeking manipulation of the norms, disruptive behaviour and exclusion. My DD really suffered because she was good at having empathy for the girls concerned but not good at protecting herself from being a target for their attention seeking self esteem building manipulation, something that others joined in with out of self preservation. She is now at another school and cannot believe how "normal" the atmosphere and social norms are. One teacher responsible for pastoral care said she the stories she was hearing made her weep at night and she has not experienced anything like it in thirty years.
The school really did try to proactively support those girls, my only criticism would be that perhaps their mental health problems meant that some girls did not experience the same boundaries on their behaviour as others, and a focus in terms of encouragement, that left others feeling discouraged and unprotected. And that then when the results started to look threatened there was a kneejerk reaction which wasn't helpful to anyone.
There was absolutely no way we could have known that a cohort like that would appear at the school though.