British TV series about shooting at school [closed]

I remember watching a series several years ago (maybe 4 - 5 years) on British television, but I missed one episode so I never really got the gist of the ending, and this has led to it being an itch-to-scratch.

Trouble, is I can't remember the name. But I do recall a few scenes - so, spoilers ahead!

Several kids go into the school. The revelation/the beginning of the shooting is initiated when one of the boys makes a remark to a girl about seeing his 'big gun'. She gives him a disgusted retort, before the boy pulls out the real gun.

There are a few scenes in a bathroom where a child is shot dead. It's later revealed there's also a bomb in the ventilation system.

A black boy who was involved in the plan committed suicide by cutting his elbow veins. It's later revealed that he didn't want to participate.

The boys in the shooting meet up and have a discussion during which one is reluctant. One of the boys makes a remark about not pussying out now, and how this is their chance. There's writing on a chalkboard behind him.

The ending involves a tactical team going to breach a laboratory-style room and shooting an armed child as the detective finds out that the boy shot in that room is innocent.

The story is sometimes narrated through tapes left by one of the main shooters.