So it was marketed as a contemp, not as a mystery or thriller? (That's me still figuring out this mixed genres thing.)
Well, in the UK the YA market is kind of small (obviously we're a small island, but it's small even compared to the amount of people living here) and we don't really get enough thrillers or contemp to catagorise them seperately. Some people might consider them different, but it's more SF/F, paranormal [romance], historical, and contemp (inc. thrillers, romance, and realistic fiction in general).
But one of our two biggest book chains, WHSmith, put it in their featured books section, I remember, and that section in itself is only about 8 books a month, so if you look at it like that, it was considered one of the top eight YA/children's books in the UK in that month.