This is a single-blind two armed cluster randomized controlled trial (cRCT) aimed to determine the effectiveness of an integrated blended intervention (online a face-to face sessions) to reduce suicidal ideation (primary outcome). Additionally, the effect of the program on the following secondary outcomes will be assessed: suicidal attempts, depressive symptoms, hopelessness and problem-solving skills. On the other side, given its probable role as underlying mechanism in SI reduction, the effect of the intervention on emotional regulation will be assessed. Moreover, the study will evaluate whether the effects of the cRCT on primary and secondary outcomes remain at post-intervention and at 3-month, 6-month and 12-month follow-up.

1. Study design. This is a single-blind, two-armed randomized controlled trial evaluating changes in primary and secondary outcomes post-intervention and follow-up. The two arms will be: 1) The intervention, which comprises eight internet-based modules of CBT delivered over 10-week, and four 45-min face-to-face psychotherapeutic CBT sessions; and 2) Control group (Treatment-As-Usual).

2. Participants. Schools with secondary education (Grades 9-12), mixed-sex, located in three different cities in the South of Chile (Talca, Linares, Curicó) with three classes per year.

3. Sample size. It is expected to recruit at least ten schools per arm, allocated on a 1:1 ratio. Each arm should include 140 eligible students. This sample size takes into account a loss at follow-up of 20% of the students. Considering a 20% of school rejection at the recruitment stage, it is expected to recruit 12 schools. Studies of similar school-based interventions report effect sizes of at least 0.4 standard deviation using symptoms questionnaires as proposed here. The results previously observed by Hetrick and colleagues and the clustersampsi command in Stata Software will be used to estimate the number of clusters in two arms, using the following command: clustersampsi, samplesize mu1(X1) mu2(X2) sd1(Y1) sd2(Y27) m(Z) rho(R). Where X1 = mean in arm 1; X2 = mean in arm 2; Y1 = standard deviation (SD) in arm 1; Y2 = SD in arm 2; Z = number of children per school on average (harmonic mean) (n = 14); and R = intracluster correlation. The two arms will be balanced for school size.

4. Process

   4.1. Recruitment. The potential participants will be schools with secondary education from three cities in Chile. The schools will learn about the study through invitations that will be made to their institutional email and social networks. Eligible schools will be invited through letters sent to the authorities who will sing a letter to participate in the study.

   4.1.1 Enrollment Strategy. This research project will be disseminated after the approval of the Scientific Ethics Committee. At the beginning of the academic year, a letter with information about the study and the written consent will be sent to all parents or main caregivers. To select the participants, all students attending to 9-12 Grades will be invited to participate in the study, and the students will be asked for a written assent. All consented/assented students will be surveyed with screening questionnaires during the second month of the academic year (C-SSRS, PHQ-9; CAPE-P-15). All students attending 9-12 grades in selected secondary schools and scoring > 3 points in the Columbia Severity Raging Scale (CSSR-S) will be invited to participate in the study. From previous data, it is expected to find 10% of students with suicidal thoughts. Students with intellectual disabilities and communication difficulties due to language will be excluded. Students with one or more suicide attempts during the last month (>11 on the PHQ-9 scale) and students with psychotic symptoms (>1.50 in the CAPE-P-15) will be also excluded. These students will be referred to Primary Care Centers and will be invited to use the intervention "Take care of your mood" ("Cuida Tu Ánimo; a universal intervention that shows beneficial effects for the prevention of depressive symptoms) available for free at the following link (http://cuidatuanimo.net), where students will find helpful information to prevent depressive symptoms. This intervention has been developed by a research team with whom Dr. Daniel Nunez collaborates.

   4.1.2. Informed consent will be requested from students. The students of the selected schools will be invited to participate in informative talks about the study. Once the study, inclusion and exclusion criteria have been explained, a description of the screening instruments, their importance, and the selection process will be provided to interested parties. The informed consent will be sent to those accepting participate by emails and signed digitally through Qualtrics. Qualtrics is a platform that allows information to be collected online, with high-security standards, and that also can facilitate the digital signature through a cell phone or tablet (using the finger) or through a computer (with a mouse). This platform will collect data of screening instruments: suicidal ideation (SI) and suicide attempts, depressive symptoms, and psychotic symptoms. These data will be used for the study eligibility analysis. All who report any level of SI during the previous month (score > 3 in the C-SSR) will be invited to participate in the following step of the study.

   Informed consent will cover both general screening and baseline measurements and follow-up during and after the intervention.

   4.2. Evaluation procedure. Self-report questionnaires assessing primary and secondary outcomes (SIQ; BDI-II; BHS, STAI-X, SPSI-R, ERQ-CA, CBTS), will be administered at baseline (one week before the start of the intervention), at post-intervention, and follow-up (3, 6 and 12 months).

   4.3. Randomization. Randomization will be performed once all schools are recruited. Schools will be randomly assigned to either group with a 1:1 allocation as per computer-generated randomization. An independent statistician will perform the randomization. Each arm should include 140 eligible students. This sample size takes into account a loss at follow-up of 20% of the students. Considering a 20% of school rejection at the recruitment stage, it is expected to recruit 12 schools. The two arms will be balanced for school size.

   4.4. Blind condition: By the nature of the intervention, the participants are not blind with respect to the group allocation (intervention or control group). The evaluations will be carried out through self-report questionnaires sent in electronic format, and their evaluation will be automatic, without human action. In data analysis, the statistician will be blind to the intervention group of participants.

5. Study duration. From its recruitment and enrollment phase to the last evaluations (follow-up), the entire study will last approximately 18 months.

   Participants will be enrolled once accept and consent to participate in the study. Once enrolled, the intervention will begin the following week to reduce, as much as possible, the possibility of variation in the results of their initial measurements between the previous evaluation period and the beginning of the intervention.

6. Intervention. The internet-based component: The intervention has been described elsewhere (Robinson et al., 2015; Hetrick et al., 2017). It comprises eight modules of CBT delivered over a 10-week intervention period. Each participant can access to a personalized web page accessed via secure login. The program will be administered at schools by trained school psychologists (1 psychologist per school). The authors will deliver the training of the program. Once each module was completed in the psychologist's presence, participants can access it from home, 24 hours a day. For safety reasons, the program does not have social networking functions. Standard CBT approaches are delivered in the intervention, with a specific focus on SI and behaviors. The topics covered by the eight modules are: engagement and problem identification, emotional recognition and distress tolerance, identification of automatic negative thinking, behavioral activation-help seeking, behavioral activation-activity scheduling (including relaxation techniques), problem-solving, and cognitive restructuring and a wrap-up session. This content is delivered via a series of video diaries by young people as main characters, with an adult 'host' character highlighting the CBT contents that had been presented in the context of the video diaries. There are two activities per module. The site has a message board through which the participant could communicate with research therapists (clinical psychologists) who also check completed activities and respond with personalized but standardized messages. Finally, a series of fact sheets cover a range of related topics, including managing SI, plus downloadable relaxation MP3s.

The face-to-face component: Traineed psychologysts will deliver four psychotherapeutic CBT-based sessions (45-min), designed during the formative work (year 1). These sessions will be conducted by school psychologists previously trained, and will help the students in three aspects: i) close support and monitoring in case of an increase of symptomatology; ii) motivation to persist with the intervention; and iii) support the interaction between the students and CBT. The final aim of the inclusion of this component is to maximize the effectiveness of CBT. Two clinical psychologists will supervise the school psychologists (each supervisor will be in charge of 5 schools).

6.1. Control group. Students will receive the Treatment-As-Usual (TAU). It consists of identification and assisted referral to a primary care clinic where trained psychologists will assess the symptomatology and propose a course of actions, from initiation of psychotherapy to referral to a General practitioner to initiate medication if needed.

Inclusion Criteria:

• Secondary students aged 13-18 years
• High scores in suicidal ideation, last month (score > 3 in the Columbia Suicide Severity Rating Scale (C-SSRS; Posner et al., 2011)
• Fluent in Spanish language

Exclusion Criteria:

• Suicide attempt(s), last month
• Severe depressive symptoms, assessed by the PHQ-9 (> 3 points)
• High severity of psychotic symptoms assessd by the Community Assessment of Psychic Positive-scale (CAPE, P15, Capra et al., 2013) (cut-off= 1.47, as suggested by Bukenaite et al., 2017)

The Outcome Assessor will be unaware of which treatment arms participants have been assigned to El evaluador será ciego a la pertenencia de los participantes al grupo control o de intervención