Here is the description of The Girl With All The Gifts that appears on its Amazon page:
Melanie is a very special girl. Dr Caldwell calls her "our little genius."
Every morning, Melanie waits in her cell to be collected for class. When they come for her, Sergeant keeps his gun pointing at her while two of his people strap her into the wheelchair. She thinks they don't like her. She jokes that she won't bite, but they don't laugh.
The Girl With All the Gifts is a groundbreaking thriller, emotionally charged and gripping from beginning to end.
I was lucky enough to have read the book on the recommendation of a friend, and didn't even know that much going into it. It's likely you've already been able to suss out at least a little bit of the "twist" the occurs relatively early in the novel, but the less you know going in the better.
The trailer for the film adaptation, on the other hand, gives just about everything away. There is no tease. No small build. It just explicitly tells you, while showing you, what's going on. I questioned whether I should even embed it in this post, but it's not like the trailer isn't everywhere this week anyway.
A few people haven't pointed out that it's likely impossible in the current pop culture climate to sell this film without giving away the films central conceit, which is fair, but it's still a bummer. If you can, go read the book at your earliest possible convenience, and then watch the trailer. I think you'll like both.