And having listened to Mr. Sutter reiterate the overwhelming agreement among scientists regarding humanity’s role in global warming in answer to another classmate’s questions — “What if we’re not the cause of it? What if this is something that’s natural?” — Jacynda texted the classmate one night using an expletive to refer to Mr. Sutter’s teaching approach.

But even the staunchest climate-change skeptics could not ignore the dearth of snow days last winter, the cap to a year that turned out to be the warmest Earth has experienced since 1880, according to NASA. The high mark eclipsed the record set just the year before, which had eclipsed the year before that.

In woods behind the school, where Mr. Sutter had his students scout out a nature trail, he showed them the preponderance of emerald ash borers, an invasive insect that, because of the warm weather, had not experienced the usual die-off that winter. There was flooding, too: Once, more than 5.5 inches of rain fell in 48 hours.

The field trip to a local stream where the water runs neon orange also made an impression. Mr. Sutter had the class collect water samples: The pH levels were as acidic as “the white vinegar you buy at a grocery store,” he told them. And the drainage, they could see, was from the mine.

It was the realization that she had failed to grasp the damage done to her immediate environment, Jacynda said, that made her begin to pay more attention. She did some reading. She also began thinking that she might enjoy a job working for the Environmental Protection Agency — until she learned that, under Mr. Trump, the agency would undergo huge layoffs.

“O.K., I’m not going to lie. I did a 180,” she said that afternoon in the library with Gwen, casting a guilty look at her friend. “This is happening, and we have to fix it.”

After fleeing Mr. Sutter’s classroom that day, Gwen never returned, a pragmatic decision about which he has regrets. “That’s one student I feel I failed a little bit,” he said.