Students will participate in a school competition to win points for their class for a 10-week period. In every HIIT session the heart rate will be tracked to measure the exercise intensity and also it will be used to win points for the competition. Every class will receive the same number of points as the sum of the minutes of exercise at a vigorous intensity (i.e., ≥85% max. heart rate) of each participant. This fact means that the physical fitness is irrelevant to the competition aspect, which will only consider the effort of the participants.

Lower body musculoskeletal fitness will be assessed through the standing long jump test, which is considered a general index of muscular fitness in youth. Participants will jump as far as they can from a starting point behind a marked line.

Waist circumference will be measured in centimeters using a non-extendible steel tape.

Height will be measured in centimeters using a portable stadiometer.

Weight will be measured in kilograms using a portable digital scale.

Weight and height will be combined to report BMI in kg/m^2).

Chronic stress levels will be analyzed in a sub-sample of students (n = 200). Stress can be accurately assessed by determining the accumulation of cortisol within the hair shaft using a sample taken from the vertex at the back of the head. As human hair grows at approximately 1 cm per month, each cm reflects the stressors experienced in the corresponding month. Cortisol concentrations will be determined from the 3 cm hair segment closest to the scalp, representing cumulative stress levels over the previous 2-3 months. The intra-individual stability (r = 0.68-0.79) and validity of hair cortisol as a retrospective measure of stress are well-supported in the adolescent literature.

Perceived stress will be self-reported using the European Spanish version of the Perceived Stress Scale, which is designed to assess the degree to which situations in one's life are stressful. Participants will be required to respond to the 10-item questionnaire in relation to the previous month. Responses are scored on a 5-point scale ranging from 0 'never' to 4 'very often' and then summing across all scale items. Higher scores indicate a greater degree of subjective stress experienced by participants

Sleep quantity and quality will be objectively measured using data provided by wrist-worn accelerometry. Sleep will be analyzed according to the algorithm developed by Sadeh. Sleep quality will be represented by sleep efficiency (SE), sleep onset latency (SOL), and time awake per hour after sleep onset (TAPH).

The Spanish version of the self-reported Pittsburgh Sleep Quality Index (PSQI) will be administered. This standardized clinical instrument covers a broad range of sleep quality indicators. It comprises 19 items evaluating seven components: sleep duration, sleep disturbance, sleep latency, daytime dysfunction due to sleepiness, sleep efficiency, overall sleep quality, and the use of sleep medication. Using a predefined formula, the PSQI score will range from 0 to 21, with higher scores indicating poorer sleep quality.

Intrinsic (e.g., "I exercise because it's fun") and identified (e.g., "I value the benefits of exercise") motivation will be assessed using subscales from the Spanish version of the Behavioral Regulations in Exercise Questionnaire. Responses will be scored on a 5-point scale ranging from 0, "Not true for me," to 4, "Very true for me."

On-task and off-task behavior will be assessed during regular classroom activities using established observation methods. Trained research assistants will conduct 30-minute classroom observations (starting 5 minutes after students enter). They will assess six randomly selected students per session (5 minutes per student). Observers will record student behavior every 10 seconds using an observation sheet, switching to the next student after 15 seconds, and repeating this process until all students have been observed 20 times. Teachers and students will remain unaware of which students are being observed.

On-task behavior includes actively engaging in academic tasks, such as reading, writing, or completing assigned work, as well as passively listening to teachers or peers. Off-task behavior includes motor (e.g., walking around), verbal (e.g., unrelated discussions), and passive (e.g., staring into space) actions unrelated to the task.

Systolic and diastolic blood pressure will be measured using an Omron M3 Blood Pressure Monitor with a cuff size appropriate for each participant's upper arm circumference. Participants will sit quietly for 5 minutes, after which three measurements will be taken at 5-minute intervals. The analysis will use the average of the three readings.

Posture will be assessed using the UPRIGHT Go Posture Training Device (Upright Technologies Ltd., Yehud, Israel). The device will be positioned at the cervicothoracic junction, just below the first prominent spinous process, using the Upright Necklace. It will record posture during an entire classroom session, reporting the percentage of time spent upright versus slouched. The device pairs with a smartphone app, enabling progress tracking toward improved posture.