The biggest problem with "The Professor" is the lack of a plot. Things happen, certainly, but there's no conflict to drive the story forward, nothing to really keep you interested in it.
According to the back cover, the plot is that he meets Mademoiselle Frances Henri but headmistress Reuter is trying to get in the way. That could have been the plot and made for a pretty good conflict, and maybe that would have made things a bit more interesting. Instead, we don't hear a word about Mlle Henri until she makes her first appearance, a few pages short of the middle of the book... and then nothing much happens. He's impressed with her English skills, but nothing really happens, then she leaves, he tries to find her (semblance of a plot, finally!) and then he does find her and then nothing really happens and then he switches jobs and nothing really happens and then he asks her to marry him ... and then nothing really happens and then it takes twenty pages for them to live happily ever after, in the same sort of drawn-out (not to mention very convenient) ending that you feel is about to finish a number of times, like The Lord of the Rings: The Return of the King (Platinum Series Special Extended Edition), except it doesn't, it goes on. And on. And on. And doesn't really get to the point. The very ending itself is also a bit bland and left me with a "meh" kind of feeling.
One of the things that kept bugging me was how much French there was. Common language back in the day, and we know Charlotte Brontė studied in Brussels, but does she have to show off her French skills on every other page? A word or short phrase here and there, fine. Actual bits of dialogue longer than a sentence which she gives no translation or even some allusion to what's being said so that we who do not know French by heart? No! I did French in school for three years, and I didn't do very well. I have no wish to start taking French lessons again just to be able to understand bits of a book that's written in English by an English author! I wouldn't have been particularly pleased had she written those bits in German instead, even though I technically have a better grasp of that language, because I still find it rude and obnoxious to the readers who aren't "in" with the author.
Frances Henri we never really get a very clear image of, aside from what Crimsworth is telling us (the novel being written in first-person singular narrative), but that's never quite enough to entice me into properly cheering him on. Possibly because Crimsworth himself comes across as rather self-absorbed and superior, so I really have problems liking him, because he's just not particularly likable as a character. Crimsworth is just stiff and dull and is always superbly in control of his features so that none might interpret him correctly, and he likes it that way. He's about as cuddly as a fridge freezer.
Then there's the issue of attitude. Plenty of negativity expressed against Catholics and of different nationalities in general (it just so happened that a lot of them were from Catholic countries). I'd call it xenophobic, because Brontė is stereotyping people a lot, and a lot of it is very negative.
Inspired by her stay in Brussels, and her crush on Monsieur Heger, I can't help but feel "The Professor" is nothing but Charlotte's romantic fantasy about herself together with him, living happily ever after in a sort of perfect dream world. In real life, she couldn't get him, but in her book, with a few changes to names and circumstances, they can be together. The book feels naive, somehow. Immature, perhaps. Oh well, at least it's relatively short: 199 pages (in the edition I read).