"No Gun to the Victor" a.k.a. "Consumer's Report", a short story by Theodore R. Cogswell, also the answer to this old question among others; first published in Imagination Science Fiction, October 1955, available at the Internet Archive. You may have read it in the 1976 anthology You and Science Fiction published by National Textbook Company. If not that, then maybe one of these.
Bringing in the mail:
It was Saturday so Alan had to go out and get the mail. Just as the letter carrier's tank clanked away, he got his cousin Alf to man the front door turret and went
zigzagging down the communication trench that led to the street. As he reached cautiously up to open the small door in the bottom of the armored mail box, there
was a sudden crack from across the way and the whine of a near miss sent him tumbling back into the slit trench. A moment later there was a coughing stutter as Alf opened up with the fifty and pounded a burst into the tungsten steel shutters of the house across the street. Alan jumped to his feet, dumped the mail out of the box, and then made a quick dive for safety just in case Alf's fire hadn't completely discouraged the Higgens kid.
The "football game":
The field lights were already on and the stadium a quarter full when Alan slipped into the locker room. He was ten minutes late and had to hurry with his dressing, but for once the coach didn't bawl him out. Coach Blauman didn't even notice him — Coach Blauman had troubles of his own. He was over in one corner telling them to Dan Ericson, the sports reporter for the Tribune who covered most of the high school events.
[. . . .]
Alan was lugged off the field at the end of the second action with a gash in his head that took six stitches to close. During the rest of the quarter he sat woodenly on the bench in the players dugout. A telescreen at the far end was following the play but he didn't lift his head to look at it. He looked like a clockwork manikin that had been temporarily turned off.
He was sent back in just before the end of the half. Illegally, it is true — the enemy had already received credit for one wounded, and according to NAA rules he was supposed to be ineligible to continue playing. Blauman didn't have any choice, however. The last drive of North's had torn up his
whole center and he didn't have much left in the way of reserves.
As Alan trotted out toward the foxholes that marked his side's last stand, he passed stretcher bearers bringing back the dead and injured from the last play. Most of them were wearing the green helmets of Marshall. The PA system announced the substitution and there was a feeble cheer from the
Marshall side of the stadium.