Background: The incidence of mental health issues in children is increasing worldwide. In Chile, a recent surge in reports of deteriorating mental health among school populations and an increase in complaints related to poor school climate have been observed. Physical activity, specifically active breaks in the classroom, has shown positive effects on children's health. However, evidence regarding its impact on mental health and school climate in children is limited.

Objective: This article outlines the design, measurements, intervention program, and potential efficacy of the Active Classes + School Climate and Mental Health project. This project will assess a 12-week program of active breaks through guided videos with curricular content in the school classroom, and its effects on mental health and school climate as its primary contributions. Additionally, it will measure physical activity, physical fitness, motor competence, and academic performance in students aged 6 to 10 years in the Biobío province, Chile, as secondary contributions.

Methodology: It will be performed a multicenter randomized controlled trial involving students in the 1st to 4th grade (6 to 10 years old), encompassing a total of 48 classes across six schools (three intervention and three control) in the Biobío region, Chile. Video-guided active breaks will be implemented through the Active Classes; web platform, featuring curricular content, lasting 5 to 10 minutes and of moderate to vigorous intensity physical activity, twice a day, Monday to Friday, over a span of 12 weeks.

Expected Results/Discussion: To our knowledge, this will be the first study in Chile to evaluate the effects of incorporating video-guided active breaks with curricular content on mental health variables and school climate in schoolchildren. Thus, this study contributes to the scarce evidence on the effects of video-guided active breaks on mental health variables and school climate in schoolchildren worldwide. Additionally, it will provide crucial information about active teaching methodologies that have the potential to positively contribute to the well-being of students, thus addressing the problems of mental health and climate in Chilean schools.

Inclusion Criteria:

- a) Regular male or female students in primary education, from first to fourth grade, at a public school in one of the three provinces of the Biobío region, willing to participate and with parental consent.

b) Students spend at least 38 hours per week in classes (6.5 hours daily) and have at least two short breaks (10-15 minutes) per day.

Exclusion Criteria:

• a) Students with a medical diagnosis of spinal pathologies, vertigo, or uncontrolled hypertension.

  b) Students with severe intellectual disabilities prevented them from following the program instructions.

  c) Students participating simultaneously in another project with similar objectives.

After schools agreed to participate, they were randomly assigned using a sequence generated by the IBM SPSS statistical package, with concealed allocation to one of two groups: 1) the Experimental Group (n=3), receiving a 12-week program of video-guided active breaks with curricular content in the school classroom; and 2) the Control Group (n=3), receiving the same intervention as the experimental group after the final data collection. A simple stratified randomization sequence by sex was used to achieve balance between males and females in both groups. The randomization sequence will be concealed through sequentially numbered, opaque, sealed envelopes, conduct by a project investigator with no contact with schools and participants. Significant differences in the quality of life, motor skills, and academic performance have been found in studies with similar samples.