Hey Louisa, hope you're well.
Wow. I thought this was really good, you've got a solid handle on a really gritty interesting world. Your characters all have unique flavour and the environment is threatening and believable. There's a good pace to your writing as well, it crackles along very nicely. I think the highest compliment I can give is that I don't really have much interest in the crime genre, but you kept me reading and kept me hooked.
It reminded me a lot of Brick, have you ever seen that film? Its got Jospeh-Gordon Levitt in and its freaking brilliant. Put me in mind of that, but with a totally fresh British spin on it. I like how you've taken the world of a high school and made school kids as complex and layered as characters in a noir novel.
The only things I'd point out is that the chaper headings are a bit confusing if there's no mention made of the fact that Sharron is pregnant from the get go. When do we find out that she's pregnant?
Looking forward to reading more. You're definitely a writer to watch.