Hogfather. Really, the assignment was to read a book by a living, non-American author, with a few blocked books (looking at you, Harry Potter), but I asked the teacher for a recommendation and he gave me the book. Like I still have it on my shelf. It kicked off a frenzy of reading as I tore through the series.
It is a terrible place to start in, though. It’ll hook you, yes, but it falls in the middle of several threads.
HHGTTG - not required academic reading, but absolutely my favorite book from my HS years, and still is. Best book I had to read, probably All Quiet on the Western Front. (And Crime and Punishment and le Morte D’Arthur and Animal Farm and …)