Not necessarily fatal -- Anne of Green Gables managed to go on for several chapters after her graduation -- but I put down one work and never came back, and I was seriously tempted to do so with the other.
The thing is that Mark Twain's rules:
In both of these books, the main characters' main goal while in school was -- to complete school. And when they stopped, they stopped. Neither one got jobs with any comparable purpose or anything else. If I wanted to read about overeducated twits living a life of idle hedonism and so be bored out of my skull, I'd read mainstream fiction.
(Anne had a chance to continue her education after. Perhaps that was the difference.)
If the education had been instrumental, if the characters had taken it and run with it, it would have worked. But as it was, the story stopped dead.
Which is why, no doubt, most school stories end before or at graduation. Wise of them.