a set look about the mouth

[majlo]

"Here comes Frank Minot, looking as solemn as a judge," cried one, as a tall fellow of sixteen spun by, with a set look about the mouth and a keen sparkle of the eyes, fixed on the distant goal with a do-or-die expression.

What exactly does the text in bold mean?

The text refers to sledging. Can the spun by mean that the one went by the people looking at him spinning?

 

[majlo]

Actually, I can't grasp the meaning of the whole paragraph. Could you please rephrase it?
Thank you.

 

[foxfirebrand]

"A set look about the mouth" indicates a controlled, unflinching look of determination. The young man has his emotions under control and is focused on the task at hand, undistracted by other issues.
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[Thomas1]

"Here comes Frank Minot, looking as solemn as a judge," cried one, as a tall fellow of sixteen spun by, with a set look about the mouth and a keen sparkle of the eyes, fixed on the distant goal with a do-or-die expression.

What exactly does the text in bold mean?

The text refers to sledging. Can the spun by mean that the one went by the people looking at him spinning?

This thread can be of value.

In Polish it would more or less be:
"I oto pojawia się Franek Minot, z miną poważną jak u sędziego", ktoś krzyknął, kiedy wysoki szesnastolatek pojawił się na horyzoncie, pełen werwy i z charakterystycznym błyskiem w oczach, utkwionym gdzieś daleko, z grobowym wyrazem twarzy.

I think it's an excerpt from a handbook for highschool learners of English, isn't it?