I think you already thought of a lot of great possibilities, but here some more I thought of.
I think you need to realise these teens just came from a (probably very) strict prison system, with rules, rituals, and orders. Some people automatically fall in line and practically follow any command issued at them (up to a point that differs for anyone). Additionally, it could very well be that the teen likes these rules and enjoys to follow them. Assuming they came from an actual violent background, they probably did not have a lot of stability in their lives. To then suddenly have a system in order can be very refreshing, including knowing what you are dealing with all the time.
Another point to consider is the type of crime the teens got thrown in jail for. Did they had one offence too many? Were they a serial killer? A gang member who willingly takes a bullet for a fellow gang member, but who murdered a rival gang in cold blood? Did they murder someone? If yes, was it premeditated or a crime pasionel, or perhaps a robbery gone wrong? Were they an accessory to the crime, or the actual perpetrator? Crimes can be done by so many different types of people, even conventionally good people (ignoring the crime).
For example, the girl who murdered her cheating boyfriend and the guy he cheated with was a member of the Chess club, ran marathons in her free time, and whose favourite subject was History. The sudden seething rage she felt was blinding, and she has never felt that way before or since. In prison she was timid and mostly kept to herself, intimidated by the more ruthless teens. However, after all that has followed after the murder, she still feels not one ounce of remorse. Oh, she feels sorry for their families and symphatises with their lose, but she does not regret the deed aside from the fact she is now incarcerated. Now she is stranded on a scarred and unfamiliar world with only a pocket knife. She can run fast, but she has never had much strength in her; Even before the blast wild animals were difficult to haunt down, and they had rifles! So, she knows she need other people, and decides for now to stick with the others. She will keep to herself and trying not to stir up any trouble in order to survive. The feeling of the pocket knife on her belt, and the thought that she can outrun most people comforts her. Okay, that might be some initial reason for her to stay with the group, what about staying? Well, they use her quickness in order to lure animals in hunting her, while in reality she leads the pray to the Ambushers, where then the Enforcers kill the animal off. She is known in the camp as withdrawn, which for other people lead to problems and usually banishment. See, the withdrawn people were seen as snobby or even them not trusting the group, and eventually were pushed out. However, she does talk about her favourite subject. Small bits of trivia here, and interesting tidbits there, and she is more known as someone who likes to keep to herself, but can share a good History story, than an outsider who is going to cause problems down the line. No one really notices she never actually tells anything about herself, and only stories from history, and she is grateful for that. At some point a guy higher up in the chain of command learns of her historic knowledge, and utilises this in order to learn more about their immediate area, for instance locations of destroyed cities that might still have valuable goods in them. This information gives her the power to slowly rise higher up, until she is in a position where she has a nice bit of power, but nothing that any higher up could get threatened by. She tries her damndest to stay in this position, even if other try to give her more power.
Okay, I got a bit carried away in that example, sorry about that. An other (shorter!) example could be the gang member. He already was raised in an environment with rules and a pecking order. He gravitates towards this, and he wants his loyal brothers who will willingly take a bullet for him around him, while they are all trying to further the goals of the gang and trying to move up within by earning their respect. Of course, he does not always realise that his brothers might not be as loyal to him as he is to them, not even in his old gang back when he was free, which will bite him in the ass at some point. If he lives, he might become incredibly cynic and ruthless, or he might become a broken man, uninterested in his fellow humans.
I might have an answer to your question why they prioritised fucking over food gathering. These teens just came from the prison system. I assume that shortly before launch they have eaten something, and that while present in the system they got meals on a regular basis. In other words, the teens are not hungry, and why care about food when you are not hungry? However, they are horny right now, and are closer to the opposing sex than they have been for a long time. So, you know. Immediate gratification versus long term thinking.
Finally, whatever you write, think of what you would do yourself. Or someone you know would do. I would have joined the group without protest, I think. Safety in numbers is a reason, but I am also not a good figther and the worst leader, so that is not really an action I would consider taking. If I do, it will definitely lead to a fight. I get killed or incapicitated. It is quite hard to hunt without any know-how when you are healthy, think of how to do it with a broken arm that cannot be set! You will most likely die. Now, these leaders could very well have shown this initial toughness in order to make sure the group stays together. They might take a bigger share of whatever gets gathered, but you are still fed as is everyone else. I would be fine with that situation. Now, it is also possible that they started good, but slowly turned for the worse. The road to hell is paved with good intentions after all. It might become so bad that everyone is slowly starving and where a simple scrap of food from the higher ups is seen as generous for certain ... services performed. Now, that is not a situation I want to find myself in, but cults exist and happen for a reason. Of course, there is always the possibility of the leaders being completely ruthless and unfair. If they are too ruthless, they will be quickly disposed of by the group because of their danger to the group, and someone else will take their place. Possibly trading one evil for another? If they are ruthless, but too powerful, then yeah, I would try to escape. But it is easier to escape succesfully when you have friends and supplies, something you do not have on the first day. So, I think it is a good idea to stay with the group, and see what will happen and plan from there.
Okay, this has become a bit long, sorry for that. But, hopefully you find it useful :)