When the choir first appears, they are in formation. Jack is a drill sergeant. He will not allow them to fall out. When one of the choir members actually does seem to almost faint, then, and only then, does Jack give them permission to stand at ease.
Jack is high strung. He is controlling. He dictates to his choir. He addresses them with formality. The first signs of his aggressive behavior are shown by the way he commands the choir.
[The choir] was a party of boys, marching approximately in step in two parallel lines and dressed in strangely eccentric clothing.
No doubt, Jack has his troops in order. They march in step along the beaches. Jack is domineering and controlling of the choir from the beginning of the novel:
The boy who controlled them was dressed in the same way though his cap badge was golden.
When the choir begins to scatter, Jack "shouted at them."
'Choir! Stand still!'
To Jack's command, the choir obeyed:
Wearily obedient, the choir huddled into line and stood there swaying in the sun. None the less, some began to protest faintly.
When one boy "flopped on his face on the sand...the line broke up."
Jack's response is that "he's always throwing faint." Clearly, the choir is obedient and afraid of Jack.
As the choir is the last to appear after Ralph had sounded the conch shell, Ralph agrees that they will remain under the leadership of Jack:
The last boys to appear are the choirboys, led by Jack Merridew. Once assembled, the boys decide they need a chief and elect Ralph. Ralph decides that the choir will remain intact under the leadership of Jack, who says they will be hunters.