I read "Childhood's End" and "Against the Fall of Night" in 7th/8th grade (yes, that was a long time ago). They remain my favorites from his body of work. Both contained ideas that blew me away, bent my teenaged mind forever. Judging by what's being posted, a lot of the community was similarly affected by his writing.
My SF book club edition of "Childhood's End" has no print date, but my 60 cent copy of "Against the Fall of Night" is dated April 1970 for its 5th printing. Obviously, both are still on my shelves.