I read it in the sixth grade, and it really really unsettled me. I grew up at the tail end of the Cold War, when this kind of scenario was still possible (thought not half as likely as when the book was first written), and it freaked me out deeply. I re-read it again as an adult and found it even MORE unsettling, thanks to little hints and cues I'd missed as a child. Loomis even initially seems a lot more sinister when you read the book as an adult than as a child, among other things.
When I read it in school, one of our assignments was to re-write the last chapter however we thought it should have ended, so I had Anne shoot Loomis in the head and toss his body into the deadness, which then gave ME nightmares about zombies in radioactive-land. There's something very inescapably horrifying in Ann's simple descriptions of the 'deadness', and how she sums it up with "I don't go out there." The imagery is straightforward but somehow even more chilling because of the matter-of-fact way it's presented.