What is going on with my chicken?

[Blackberry18]

So, I have eleven 15-week old chickens with a 6 older hens. They been together for several weeks and everything has been fine. A bit of aggression, but that's normal. However, a couple days ago, I noticed that one of my Partridge rock pullets looked like she was missing neck feathers. I ignored it at first, but now it just looks odd. She walks around looking like a rooster about to attack, her feathers all up. Now, I'm not sure if she's missing feathers or they're all flattened, but she's totally fine except for that. Is it caused by the other hens, or possibly even a couple of the cockerels? Please help, how can I deal with it?

 

[Wyorp Rock]

So, I have eleven 15-week old chickens with a 6 older hens. They been together for several weeks and everything has been fine. A bit of aggression, but that's normal. However, a couple days ago, I noticed that one of my Partridge rock pullets looked like she was missing neck feathers. I ignored it at first, but now it just looks odd. She walks around looking like a rooster about to attack, her feathers all up. Now, I'm not sure if she's missing feathers or they're all flattened, but she's totally fine except for that. Is it caused by the other hens, or possibly even a couple of the cockerels? Please help, how can I deal with it?

Could be a combination of hens pecking at her and cockerel mating injury. Cockerel hold on to the neck and can pull feathers and skin leaving a bare area.
To me the first photo looks like an injury, you may want to keep an eye on it for any infection. You can apply some plain neosporin or vetericyn to promote healing. If it doesn't seem to be healing well, you may want to separate her or re-evaluate your cockerel to hen ratio.

 

[Pork Pie]

If your cockerels are only 15 weeks old, i am not too sure that they would be "allowed" by the hens to try and mount them - even if their hormones had kicked in. If you have observed your cockerels trying to mount your adult hens, then its possible that it could be a mating-related injury. In my experience, a roo will tend to hold onto neck feathers just behind the head, and not as low as the photo suggests, but a smaller cockerel may i guess.

Wyrop has given you some good advice re: treatment and i would observe your flock (particularly when you let them out on a morning and in the evening, when any cockerel shenanigans are most likely to occur) to see if you see any evidence of mating attempts.

CT