Are answers of the form "This didn't actually happen in the book (that movie is an adaptation of)" to a question "Why did X happen in the movie" acceptable?
Especially in cases where there seems to be no good movie-based answers?
Case in point: " How did Crabbe and Goyle get into the advanced N.E.W.T. class? "
The answer is: There's no explanation in the movie, and the book doesn't have them in the class so there's no canon reason.