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The paradox of Lexington High School is that while indicators of anxiety abound, so too does an obsession with happiness. A large banner from the town’s newly formed suicide prevention group, a chapter of the national organization Sources of Strength, greets students as they enter the sprawling red brick building, proclaiming: “Be a Part of Happiness.” There are close to 50 students in the group. Below the banner are remnants of their project to spread positivity. Students were asked to write down their sources of strength, which were then posted beneath the banner and on Facebook. Some named their pets or friends. One wrote: “My mom.” Another: “Trip to Israel!” A girl with green hair: “Chicken curry.”

One morning in February, students in “Positive Psychology: The Pursuit of Happiness,” a popular elective, were following up on a discussion about the psychologist Barbara Fredrickson’s “broaden and build” theory, which posits that negative emotions like anxiety and fear prompt survival-oriented behaviors, while positive emotions expand awareness, spurring new ideas, creativity and eventually building skills.

“Today, we are going to look at pretty simple ways to make it more likely that you experience positive emotions on a day-to-day basis,” Matthew Gardner told his “Happiness” students as they pulled out notebooks and pencil cases. The class discussed the benefits of exercise and eating foods that release feel-good hormones. The students also learned that smiling and being smiled at releases dopamine, which has an uplifting impact.

Mr. Gardner offered an alternative to smiling: “Our brains are not so perfect that, sometimes, if you hold a pen or pencil like this” — he held a pencil between his teeth — “you activate some of the same face muscles. You might get a little bit of a dopamine effect, too.”

Several students held pencils between their teeth to test the theory.

At one point, the class practiced laughter yoga, raising their arms slowly as they breathed in, then lowering them as they breathed out, and bursting into peals of laughter. Afterward, the students recorded changes in their pulse rate to demonstrate research from the HeartMath Institute that shows heart rates slow down and smooth out after bouts of good feeling.

“It’s not just that your heart rate goes down and you become very calm,” Mr. Gardner explained. “It’s that the shape of your heart rate is smooth and more controlled. Frustration is more jagged.”

Their homework assignment: Do laughter yoga or “smile at five people you wouldn’t normally smile at.”