Besides (as others have pointed out) some of the classics aren't that great. Then, there is the problem that it seems frequently the books chosen for a high school reader are chosen based on some perceived quality of the book rather than what is going to be enjoyable, relatable and readable by someone who effectively doesn't have any adult life behind them.
Worse they seem to be chosen without regard to length or readability. I had to read Grapes of Wrath over 3 weeks in HS (with everything else going on, and I know most students shared a cliff notes and prayed for a reasonable grade), but there are plenty of worse examples and the The Count of Monte Cristo in its 1000+ page unabridged addition was assigned my niece last year. I tend to be able to read pulp novels at 100 pages an hour, but older novels, Shakespeare plays etc, can easy bring it down to 30 pages an hour or less. Which for a 600 page book is a 20+ hour long drag if the novel isn't enjoyable.
Surely the instructors can find shorter novels which are easier for the kids to read and relate to. I remember reading Cannery Row a few years ago and wondering why instead of Grapes of Wrath it wasn't assigned, or for that matter I read "This Side of Paradise" and am to this day 100% convinced that any teacher that assigns The Great Gatsby to a teenage student over it should seriously be examined (actually IIRC when I read it, it seemed the sexual content in this side of paradise probably makes it hard to assign, but that is another discussion, along with the fact that Gatsby was initially a failure with mixed reviews, the reasons should be understood but apparently aren't. I hated the Gatsby assignment, but suspect that I would have found it more enjoyable in my 20's when I read Paradise, which is likely one of those books that is less meaningful to people in their 50's).
The bottom line, is that there are plenty of "classic" novels that are relatable to a wide audience (male & female, differing economic backgrounds, etc) and are short enough that a normal teenager can struggle through them even if they aren't enjoyable. The fact that it accepted to assign some of these book because everyone else is doing it is just a sign of bad instruction.