I listened to the File on 4 podcast about this case and thought along similar lines. There were lots of elements of all 3s histories that are quite far from mainstream teenage experience and behaviour.
SJ had poor attendance, a history of harming others (younger) to the point police were involved, a fixed exclusion from school, managed school move, worrying internet usage and drug usage. She was educated in an inclusion unit when she did attend.
ER was a socially awkward loner, whose WhatsApp messages are really chilling. Clearly not everything is ok at home if the rumours are true.
BGs history of social anxiety, eating disorders, self harm, hospitalisation for mental health issues, reduced timetable at school, arrangements for eating separately, and attending the inclusion unit were all openly discussed on the podcast by parents and teachers. BG had been estranged from her father and had had tentative phone type contact in the time leading up to the horrific crime. But it was also mentioned on the podcast that this 16 year old had 30k followers on tiktok. I don't have tiktok but that can't be average followings for a young teen? It seems so intrusive for someone grappling with such severe mental health issues, but it seems all the adults around BG were comfortable with this. SJ was described as being obsessed with BG and I wondered whether her celebrity was a factor? Knowing someone irl that others follow on a screen has kudos etc. Brianna was without doubt a very vulnerable young person. Hearing her mother talking about how pleased she was that she had made a friend in SJ and how supportive she was of BG getting a bus and meeting up with friends is just beyond heart breaking. Her mother is very articulate and makes some very difficult but thought provoking points in the podcast.
I don't think any of the above factors are in anyway an explanation or necessarily linked to what happened, but I did find it really shocking that much of what went on was described as typical teen stuff. Many of us are maybe completely out of touch as to what is genuinely typical?
This crime is different to other teen stabbings, not so much necessarily due to the victim, but due to the extensive levels of premeditation, planning, discussion and fixation of the perpetrators. Many of the sad teen stabbing deaths we usually hear about are more spur of the moment, fight goes wrong type scenarios or linked to gang and turf wars. Every single one a devastating loss. It does feel that the work needing to be done to prevent those deaths, and what potentially could or should have happened to prevent this hideous event are different maybe?