UP-A adapted as a preventive intervention. Specifically, the Spanish version of The Unified Protocol for the Transdiagnostic Treatment of Emotional Disorders in Adolescents (UP-A) adapted as a 9 sessions school-based preventive intervention. The intervention is delivered in nine weekly sessions (each session lasting around 55 minutes).

Same than experimental group (EG), after EG finishes. Specifically, classes in waitlist condition will be offered the same intervention than experimental group after EG finishes the three months follow-up.Before that, waitlist means that the classess work as usual with issues of mental health.

The Spanish Version of the Unified Protocol for the Transdiagnostic Treatment of Emotional Disorders in Adolescents (UP-A) adapted as a school-based anxiety and depression preventive intervention. The UP-A (Ehrenreich-May et al., 2016) is a transdiagnostic anxiety and depressive disorders treatment protocol that implement a set of core principles to address common factors underlying youth anxiety and depressive symptoms in an individual setting. Core modules of the UP-A include: (1) Building and Keeping Motivation; (2) Getting to Know Your Emotions and Behaviors; (3) Behavioral Activation; (4) Sensational Awareness; (5) Being Flexible In Your Thinking; (6) Awareness of Emotional Experiences; (7) Situational Emotion Exposures and (8) Keeping it Going - Maintaining Your Gains.

RCADS-30 is a widely-used questionnaire of self-reported anxiety and depressive symptoms in children and adolescents. The scale is comprised of six subscales derived from DSM-IV criteria: social phobia (SOC), generalized anxiety disorder (GAD), obsessive-compulsive disorder (OCD), panic disorder (PD), separation anxiety disorder (SAD), and major depressive disorder (MDD). All subscales except for the MDD can be summed to creat a Total Anxiety Disorder Symptoms Score. Each item is scored from 0 (= Never) to 3 (= Always), with higher scores representing more severe symptoms.

The CDN is a 16-item self-report questionnaire designed to assess symptoms of DSM-IV major depressive disorder and dysthymic disorder in children and adolescents. In this study, the two items targeting suicidal ideation were not included because of indications from the school principal and school counseler implicated in the study. The items are rated on a 3-point scale of frequency corresponding to Never or almost never (=0), Sometimes (=1), and Very often (2). The sum of all items provides an overall score with higher scores indicated more elevated depression symptoms.

This 10- item questionnaire targeted to children and adolescents assesses anxiety symptoms during the last few weeks. Participants are instructed to respond to how frequently they have felt or experienced general anxiety symptoms on a four point, Likert-type scale, ranging from 0 (= Never or almost never) to 3 (= A lot of the times or almost always). The sum of all items provides an overall score with higher scores indicating more elevated anxiety symptoms.

The Spanish version by García and colleagues was used (García et al., 2000).