I thought it was very badly written.
(Potential spoilers). In the opening episode the police entered the house and said the boy was accused of murder. The parents said 'he didn't do anything' (understandably) but they never once asked who had been murdered. That is such bad writing - if police came in and told you there'd been a murder in the local area (so close your child could be involved) one of the main things you'd want to know is who died. It could be someone you know well, or even a family member. No one ever once wondered or questioned who might be dead until it was revealed in the police interview. Such a glaring mistake.
In the second episode two police officers were standing very close to the scene of an assault and they did nothing. At the very least they would be key witnesses in a future investigation, but beyond that they are trained to step in and deal with situations like that - instead the teachers had to do it and they just stood around like it was none of their business.
Also, a real school would be closed if one of the pupils had been murdered - or at least they certainly would not be carrying on lessons as if nothing had happened. The children and teachers would be distraught. I had the misfortune of teaching in a school where a child died by natural causes and the next week or so was a complete write off - children were absent, teachers burst into tears at the meerest provocation, parents kept visiting because they felt so lost. The child's class just sat and talked for days. In a violent situation like this, there would be counsellors on site and a far greater number of police, partly because it wouldn't yet be clear that this was an isolated incident - other attacks could happen. Instead of speaking to classes individually, the police would bring everyone together in the hall and speak to everyone at once. Their aim would be to jog the memory of potential witnesses or anyone who could give them important information - literally anyone in the school could be a key part of the case. It was bizarre when the detective just walked off and had chips with his son - I know the point that they were trying to make but a detective would never do that at such a crucial point in a case. It was in no way finished.
A forensic psychologist (which I assume the person in the third episode was supposed to be) would never give a hot drink to anyone in a secure unit when there was high potential to piss them off. Asking for facial burns. A forensic psychologist would also have dealt with serious gangland thugs. A 13 year old screaming at her would have been upsetting but not scary.
I thought the final episode was interesting but maybe not entirely in the way it was intended. It seemed to imply that the family was happier without Jamie and that they'd distanced himself from him, that they mostly felt anger and disappointment rather than a sense of loss. That would be an interesting storyline to pursue - it was hinted at in previous episodes that Jamie felt rejected by his dad and it would be very interesting to see how the subtle rejections (and maybe the sense that they loved the daughter more) led to his anger and lashing out.
I don't know why people go about about the social media aspect of this story - it barely features. The emojis are mentioned but to my mind that's not much different than passing notes to bully someone - the method is different but the result is the same. From what the parents said Jamie was low-level neglected, made to feel not good enough by his family and then felt rejected by the girl. The manosphere may have played a part but that wasn't well articulated at all.
As others have pointed out, the girl is just a picture. Her friend was well portrayed - the sense of loss there was realistic - but apart from the stack of flowers at the crime scene there was very little sense of mourning a real person.