Name that book

sheriffwoody

There was a question on a game show I heard yesterday about the Shakespere book "McBeth" and it got me thinking about which books I read at school leading up to my English GCSE (2000-2001), and along with other Shakespere books and Lord of the flies etc, I remember one other book and can't think what it was called.

Very briefly it was about:

A teenager/young adult who had a knife
There had been a killing (I think) and he was questioned by the police, due to him having this knife
The police told him that they have a special substance they can pour onto the knife blade and it will show up and blood particles that have been on the blade, despite being wiped off previously and if it looked "clean".
The boy said he had nothing to hide so they should do the test.
It was a battle of nerve, who would give in first.
In the end, just before the court case the boy was involved in, his mother or father took the knife and ran it across the palm of their hand, thus cutting them, getting blood on the blade and meaning the "special test" would be null and void as it would pick up the mothers/fathers blood.

Any idea what book this is at all?