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). 

The grimdark setting should be just fine. Your setting can be whatever you want it to be. Nothing about that takes away from player agency, they can choose as they will within the world you've built. So that's fine! As others have pointed out, they themselves might not always stick to the grimdark script, but your world can keep to its themes.

The only real issue I'd flag is "meanwhile the main characters are a bunch of dumbasses sharing exactly one brain cell." There's no way of controlling how the players will want to act in their world. This one is where you start getting into a player agency issue. My suggestion is, write your world and stick to your themes. Talk to your players about the theme and vibe you're going for. But don't expect them to be certain characters in the story you've written in your head. It'll never work out that way. They all have characters and a game of their own they want to play.

I'd say to make sure you have a session zero, clarify that this is more of a serious campaign and to act accordingly. I agree with ereed though, you can never predict what direction characters are going to go. It's also good (in my opinion) to let them have their bits. If they're committed enough, chances are that they won't be doing bits in stressful and tense situations.

You know how in Mulan there's that one really fun song about fighting for a girl you love and it's really boppy and funny and everyone is having a great time? And then they are no longer singing at all in the movie? As the DM, I just bring the dark things and let the characters react. It's easy to forget that the characters are real to themselves, and a lot of people in stressful or prolonged dark situations will make jokes to keep themselves mentally and spiritually sound. 

As a DM, we can set the underlying tone (my entire party has just found a town under the control of an international terrorist organisation), and the party is responsible for keeping a tone that they are comfortable with (they are making jokes about anime boobs). It doesn't have to be constantly dark, and often if we DMs try to keep it that way, it stops being fun for the players and us.

Also, my players also share one braincell... I only have three players though, and none of them have decent perception so that one braincell is not useful to anyone.

It requires a frank conversation with your players: 'I want to run a grimdark style game, is that theming something you are all interested in playing? Would you be willing to play that type of setting? In a lot of these darker settings player characters do good deeds largely to do good and not to win, because winning is impossible. Are you all good with that?'  

If the players don't buy into the setting, there's not much you can do. You as DM can do some things, but unless the players buy into the world and setting, it's hard to maintain the theme. What is important though is to ensure that there are glimpses of light through the darkness. The characters need to be able to experience happiness, but the happiness can never last. That's how you develop a tone throughout an adventure. The TV show Angel did this superbly in one of their episodes where the main characters takes a journey to the home office of the evil corporation. The doors open on that destination and surprise, surprise it's earth. The same place he left. The hero doesn't get the anticipated victory. He's been successful, but it's far from a victory. There is no big bad moustache twirling villain to kill. Evil is everywhere, and the representative of the company claims they have no desire to do anything as prosaic as 'winning'. When the hero then asks why they keep fighting the question gets  turned around on him. If the hero can't win, why does he keep fighting. Battlestar Galactica (reboot) managed to maintain this oppressive, hard, exhausted mood in a similar way. No victory was ever a final victory. No win brought about peace. No joy was ever able to be savoured. If someone was happy in one scene, someone was about to die. 

What does this have to do with anything? Well, this is where I would say that player choice isn't always all that important. No matter how many times Strahd is defeated, he'll be back. Barovia may get a reprieve but the book makes clear that the people won't leave, because they have nowhere to go. Perhaps most cruel is what is written in the epilogue:

"The ancient Dark Powers with which Strahd forged his pact cause the vampire to re-form after a period of months—long enough for the Barovians to discover what it feels like to live a life of hope. When Strahd is reborn, the mists surround the land of Barovia once more, and the Barovians' hope turns to horrible despair. Strahd remembers the defeat dealt to him and begins plotting his revenge."

Frankly one of the most dark aspects to Curse of Strahd is the knowledge that the players ultimately fail even in success. Arguably they cause more harm because they allow the villagers that most deadly of things: hope. The NPCs of Barovia get to remember what it was like long, long before Strahd. They then of course will remember this moment when he returns to cast the lands back into the mists. The players' choices then ultimately didn't matter. Never the less, Curse of Strahd is without doubt one of the best 5e modules ever produced. The lesson then is that the setting must take precedence even above character choices, because in some settings victory won't always be possible.