Why does Jerry Renault know that "it was important to show no sign of distress" in The Chocolate War?

Jerry Renault is small, only five-nine and maybe a hundred and forty-five pounds, and he is not a very good football player, but he is determined to make the team at Trinity.  Although he is taking a beating on the field, he knows that the coach "is testing (him), and he's looking for guts".  Jerry does not want the coach to think he is a weakling or a coward, so he knows "it (is) important to show no sign of distress".

It is only three plays into the practice, and already Jerry has been sacked three times.  The first time he turns to take the ball, "a dam burst(s) against the side of his head and a hand grenade shatter(s) his stomach...engulfed by nausea, he pitch(es) toward the grass".  Although he feels as if "some of his teeth (have) been knocked out", Jerry rises back to his feet, holding on "until everything settle(s) into place, like a lens focusing, making the world sharp again".  The second play calls for a pass.  Jerry "pick(s) up a decent block and cock(s) his arm, searching for a receiver", when he is suddenly "caught from behind and whirled violently; landing on his knees, he urges himself "to ignore the pain that grip(s) his groin", determined not to let on to the coach that he is in distress.  On the third play, he is hit "simultaneously by three of them:  one, his knees; another, his stomach; a third, his head".  Despite all this, when the coach comes by, all Jerry says to him is "I'm all right".  Even though he knows that he has "been massacred by the oncoming players, capsized and dumped humiliatingly on the ground", Jerry knows that he has survived; "he (has) gotten to his feet", and he wants desperately for the coach to see that he has what it takes to be on the team (Chapter 1).