Mystery/Thriller - Plot: several adults (parents?) are found murdered and their children are missing. Read in 94-95

[Petertorp]

I read this book in English class in high school in Denmark back in 94-95. My English teacher has passed away and my old classmates can't remember neither title nor author. Please help!

Plot:
I believe we follow a murder investigation in a small community (small town or maybe a boarding school). All (or most) parents have been found dead and their children are now missing. As the investigation progresses it becomes more and more apparent that the children might be responsible for the deaths.

I specifically remember two "glimpses":
a parent (or teacher) found dead in a kids room, killed by a lethal trap disguised as a kite.
a large stack of adult magazines in a kids room, but most of the magazines are in fact about guns, weapons and traps. Only the top ones are adult magazines, to disguise his/hers true interests.

Please make my day :-)

/Peter