Students in the control class received regular class instruction during the 5 weeks. This instruction did not deviate from regular instruction that was provided to all classes, including the intervention classes. During the intervention period, control group students completed the assessments once per week. The control class is considered a treatment-as-usual active control.

The self-paced slow diaphragmatic breathing intervention provided guidance for participants to breathe at a slower pace than normal with brief, organic pauses after each inhale and exhale, and with exhales longer than inhales. Participants were guided to breathe at their own pace while following these principles of longer exhales and brief pauses after each inhale/exhale. They were invited to slow their pace when ready, both during each 5-minute session and over the course of the 5 weeks.

The guide-paced slow diaphragmatic breathing intervention comprised slow breathing with all exhales twice as long as the inhales; e.g., a 3-second inhale was followed by a 6-second exhale. Participants were instructed to breathe in sync with the guided pace. The breathing pace slowed over the 5 weeks: for weeks 1-2, the breath pattern comprised a 3-second inhale followed by a 6-second exhale; for weeks 3-4, the timing was 4 and 8, respectively; and was 5 and 10 for the last week.

The curriculum was based on three key breathing components that have been shown to reduce stress in adolescents: slow breathing; diaphragmatic breathing; and extended exhale breathing. Slow breathing entails breathing at a pace slower than normal breathing. Diaphragmatic breathing focuses on breaths starting from the diaphragm or abdominal areas, with abdominal, then lung, then chest expansion during the inhale and a slow, gradual, full release of air in the reverse direction on the exhale. Extended exhale breathing comprises breathing with the exhalation duration longer, often twice as long, as the inhalation.

Two versions of slow diaphragmatic extended exhale breathing were included in this study. For both, participants did the practice while seated comfortably and breathing through the nose, and were guided to increase the inhale and exhale durations over the 5 weeks. Students followed 5-minute videos for each session.

Number of participants completing the breathing practices

Number of participants completing the STAI and CO2TT assessments

Ease and tolerability of the breathing curriculum was measured as the average ratings on a scale of 0 to 3 (worst to best) of responses about the curriculum's ease, usefulness, and tolerability.

Ease and tolerability of the preliminary effectiveness assessments was measured as the average ratings on a scale of 0 to 3 (worst to best) of responses about the effectiveness assessments' ease and tolerability.

6-item STAI-State, short version scale. Responses are scored from 1 to 4. Final scores range from 6 to 24, with higher scores reflecting higher levels of in-the-moment anxiety.