The last one is a bit short, but after all the thread is titled short stories, otherwise it is nicely written, even though I prefer consensual stories, I could enjoy this one too.
Can't really say much about the one before that. I have no idea about darts rules, so for me it's a story about characters shouting some numbers. Besides they are drunk or at least were when they made the bet, which is a major turn-off for me.

The deadly bet gave me another idea though. A boy brags to a girl how strong he is and she's somewhat skeptical and somehow it comes to whether or not he can hang her by suspension all the way till she dies. Naturally (as in naturally for my ideas), they don't see anything wrong with pain and death, and just see it as a good fun. So they get a rope, tie one end around the girl's neck and throw the other end over a tree branch and the boy pulls it so that the girl can't reach the ground and tries to hold it as long as he can. He holds it long enough for the girl to experience a lot of pain and give him a good show of her struggles, but in the end he can't hold the rope any longer and the girl is still alive. After the girl recovers she says it's now her turn to try the same on the boy. He doesn't mind being hanged of course, but he wonders if it worth it, because even if he couldn't do it, the weaker girl, not to say weakened further by just being hanged, there's no way she can do it, so why even bother trying? But the girl says she knows a trick or something. The boy gets curious and lets the girl hang him. After she pulls him off the ground, she ties her end of the rope to something and says: "you might be stronger, but 'm smarter" and watches until he dies. The boy isn't angry at her for such cheating, but rather is amused, like when someone plays a prank on you, if it's a good prank, you end up laughing too.
For the sake of the story being more erotic, maybe they strip before being hanged so that there's no extra weight.
For the place where it happens, I imagine options are:
1) house and backyard of one of the kids (or they might even be siblings, so it would be their house and backyard then) where they were playing casually before that conversation came up;
2) playground;
3) a street or a park on their way from school;
The second and third options can be used to emphasize the casual aspect as all the other kids on the playground or passers-by on the street don't see anything wrong with kids stripping and hanging each other. But I wonder where they can get a rope though.
In the first scenario though some extra casualness can also be thrown in in a form of a parent observing the kids and feeling nothing but tenderness and adoration about their childish spontaneity. Maybe even giving them the rope when they ask for it. And then casually calling the surviving girl to dinner in the end. And/or praising the girl for her cleverness of how she outsmarted the boy, which would be especially fun if they're actually the boy's parents and the girl is just his friend.
Of course there can be also other options for locations and premises, like maybe the girl spends her summer holidays in a village with her grandparents and the boy is her cousin living there. Or they're on a picnic with their family(-ies), in which case maybe it's their parents who are friends and the kids themselves have just met each other here for the first time. Or whatever.
Also, the same scenario can work with two girls or two boys.