I'm trying to find a book I read some years ago?

I picked it up off a shelf in the school library and read the first few chapters, but put it back without noting the title or author and I've been wanting to find it again.

It was about a futuristic world where ordinary people are kept under control by a drug that they take to inhibit freethinking and creativity. At birth, everyone's skillsets for later life are placed into their minds so their future career is predetermined, and their creativity and imagination are phased out by drugs and other procedures.

The book introduced, however, a band of renegades who have secreted away a few of the children and are raising them to be normal and uninhibited. I remember there was one girl who liked to wear black nail polish and at the point I got up to she had complained about running out and needing to get hold of some more. Also they use the word 'king' as a swearword.

The edition I read had a pale green or turquoise cover and I think a picture of a person with their eyes closed, looking serene, as if daydreaming.

I have already tried putting all the keywords into search engines, but nothing going. If anyone can help me find it or has some inkling of what it might be or who be I would be very grateful.