It sounds like you may not have a lot of experience with chickens mating, not sure about that. I'll copy something I wrote a few years ago that might help you.
The rooster dances for a specific hen. He lowers one wing and sort of circles her. This signals his intent.
The hen squats. This gets her body onto the ground so the rooster’s weight goes into the ground through her entire body and not just her legs. That way she can support a much heavier rooster without hurting her joints.
The rooster hops on and grabs the back of her head. The head grab helps him get in the right position to hit the target and helps him to keep his balance, but its major purpose is to tell the hen to raise her tail out of the way to expose the target. A mating will not be successful if she does not raise her tail and expose the target. The head grab is necessary.
The rooster touches vents and hops off. This may be over in the blink of an eye or it may take a few seconds. But when this is over the rooster’s part is done.
The hen then stands up, fluffs up, and shakes. This fluffy shake gets the sperm into a special container inside the hen near where the egg starts its internal journey through her internal egg making factory.
This is typical mating behavior between mature consenting adults. This is an idealized situation, in reality it seldom happens exactly like this. There can be some running away and chasing. Force may be involved. As long as the girl squats and isn't injured it's all normal and natural.
With 17-week-olds you are not dealing with consenting adults. You are dealing with adolescents that often have no control over their hormones, especially the boys. The cockerels normally mature earlier than the pullets and are often driven mad by their hormones. The immature pullets have no idea what is going on so they certainly are not going to cooperate.
At that age most of this is not about sex either. The mating ritual is about dominance. The one on bottom is accepting the dominance of the one on top, either willingly or by force. The cockerel’s hormones are screaming at it to dominate the pullets but the pullets are not ready for that. It takes both to do their part, pullets as well as cockerel.
I'm not sure of what you are seeing but this might help explain about the head grab and some of the other stuff you are seeing or probably soon will see.