How To Maintain A Certain Tone in My Campaign

So Im running a campaign for my school's DnD Club based on classic fairy tales, and keeping with that, Im running it as a dark fantasy thing, with some horror elements sprinkled in. I was wondering if anyone had any advice on how to maintain that sort of grimdark tone without taking away from my player´s choices. What Ive been thinking of doing to solve this is something similar to the Castlevania Netflix show where the world is horrifying and dangerous and dark, meanwhile the main characters are a bunch of dumbasses sharing exactly one brain cell.  

Tell the players what you want and see if they'll agree to it.

This is going to be a tough one. No matter how dark and cool you try to make the campaign, the PCs will end up, I don’t know, adopting a bunny and then side questing to find a tiara for said bunny to wear. It can really be tough to keep any mood going consistently. 
Now, pantagruel’s advice above is really the way to go. Get everyone to agree on the idea and try to stick to it. 
But, in terms of setting your own expectations, be prepared that they may veer off into the absurd and/or comical sometimes.

Definitely communicate with the players about what you want. That's probably step one in any campaign. 

I'd say just look for ways to work what you want into what the players want. Bunny tiara? Tiara is cursed to make any who wear it fall asleep until they get a true love's kiss. That kind of things.

As long as you don't actually care if the players / party is genuinely scared or are just messing around doing whacky things in a grim-dark world it's pretty easy to do. The problem with grim-dark is that you can't actually sustain it fully long-term, or it gets kind of boring. Usually you need to punctuate it with moments of levity or hope.

There is nothing about grim-dark that means player choices are meaningless. The point of grim-dark isn't that any and every choice results in the same bad outcome. Grim-dark should be about hard choices, where you have to weigh different possible outcomes, and doing something against the "rules" might actually be the best choice. Take the Witcher as another example of a Grim-dark world, most of the situations & choices have a clear "better" and "worse" outcome for the player to choose between it's just that there is no "and every body lives" choice. And there may be times where the "better" and "worse" outcomes are unclear - a good person might make a terrible ruler it put in charge, or an overzealous guard who abuses their position might be the only one who can catch a serial killer (but for player choices to matter, the players should have the opportunity to figure out what the best decision is if they ask the right questions or complete the right side quests).