In Chapters 7-9, what leadership skills does Jack demonstrate?
I think the key point to make here is made by Golding at the end of Chapter 6. Jack is the one with the bravery and guts to lead the boys on a hunt for the beast, and, indeed, the chapter closes with the image of Jack going first:
Mutinously, the boys fell silent or muttering. Jack led the way down the rock and across the bridge.
Golding makes it quite clear that Ralph is happy to follow Jack, and that Jack is very clearly in charge of things: here, in Chapter 7, he states it openly:
They set off again, the hunters bunched a little by fear of the mentioned beast, while Jack quested ahead. They went more slowly than Ralph had bargained for; yet in a way he was glad to loiter, cradling his spear... Jack was in charge of the hunt and there would be time to get to the mountain...
Jack even receives a wound - a badge of the true leader:
He turned his left forearm for them all to see. On the outside was a rip; not much, but bloody.
Jack also knows exactly how to undermine Ralph. He doesn't speak much, but when he does, he knows how to put Ralph in a definitely subordinate place: when Ralph wants to start the fire:
'You haven't got Piggy's specs,' said Jack, 'so you can't.'
And finally, when Jack makes himself Chief, he understands the importance of making himself a real figurehead:
Before the party had started a great log had been dragged into the centre of the lawn and Jack, painted and garlanded, sat there like an idol.