The first step in recruiting participants will require approaching preschool headmasters with a detailed description of the study's aims and the benefits of participating in the SEP+ intervention for both teachers and parents. All recruited schools will further inform parents of enrolled children regarding the study. Parents will be asked to provide informed consent for their children's participation in the study. Also, they will be asked to make a choice regarding their own participation to the parent training component. Afterwards, all classrooms from a particular school will be randomly assigned to either the SEP+ intervention or the wait-list control group. Data on the SEP+ effectiveness will be gathered at pre-intervention (January 2022) and at 4-months post-intervention (June 2022) for both conditions, intervention and control, respectively. Primary outcomes related to children's social-emotional skills and risk of externalizing/internalizing problems will be assessed at both baseline (T1) and immediately after the end of the intervention (T2) based on teacher and parent ratings. A similar approach will be employed for secondary outcomes. More specifically, data on self-reported parental discipline strategies, emotion regulation/coping and stress will be gathered at T1 and T2. Also, parents will be asked to fill out measures of children's social-emotional skills and externalizing/internalizing problems (primary outcomes), as well as measures related to parenting practices, parenting stress and parental ER (secondary outcomes). Both types of outcomes will be assessed at T1 and at T2, whereas parents will report about the number of adverse events only at T2. Any adverse events associated with the exposure to the classroom curriculum and/or parent training such as deterioration of child and/or parent mental health will be monitored by the research team. If such events occur, these will be reported to The Scientific Council of the Babes-Bolyai University.

As the SEP+ is comprised of multiple interventions, monitoring procedures will be employed for all components. First, regarding the implementation of the classroom curriculum, teachers will be required to fill out a self-monitoring implementation form, which will include children's attendance rates, number of implemented classroom activities, and teachers' use of intervention methods. In addition, teachers will be required to provide access to the research team to 25% filmed classroom activities. The quality of implementation for each activity by two independent raters. The teacher training will be conducted with approximately 12-15 participants in each group. Teachers will be required to participate in all 6 training sessions. Fidelity of implementation in accordance with the facilitator's manual will be conducted for half of the sessions (from sessions 2 to 5) by two independent raters based on video recordings of these sessions. Checklists will contain items describing facilitators' ability to implement specific methods of training, as well as their skills regarding group management and ways to support teachers' use of learned strategies in delivering the classroom curriculum. The parent training will also consist of 6 sessions, which will be delivered to groups of 8-10 parents. For each parent training, group fidelity checks will be conducted for 50% of the parent training sessions (sessions 2 to 5) by two independent raters based on videotaped recordings. Similar to the teacher training, facilitators will be assessed on their ability to deliver each training method according to the manual, as well as their interaction and communication skills with parents.

Approximately 30 classrooms will be recruited for the purposes of the current study. Given an expected rate of 15-30% dropout form pre- to post-intervention and that some parents might not provide consent for children to participate, the investigators estimate that the final sample will include a total of approximately 250-300 participants. A GPower a priori sample size estimation for a medium effect size, a significance level of .05 and power of .80 suggests that 41 participants are required in each group.

Data for the current study will be collected online employing Google Forms and all items will be marked mandatory to reduce the likelihood of missing data from each set of parent- and teacher-rated questionnaires. In order to handle missing data an iterative Markov chain Monte Carlo (MCMR) method will be employed.

For the purposes of the current study two types of analyses will be conducted: 1) for the effectiveness of the universal intervention (for all children in the sample), and 2) for the indicated intervention levels (children who score above the cutoff (T score \> 67) on the Externalizing Problems score on the Child Behavior Checklist and the Caregiver-Teacher Report Form. For the universal intervention multi-level mixed models controlling for clustering effects will be conducted to test the intervention's effectiveness and moderator effects. Additionally, mediator effects will be tested employing multi-level meditation models in MPlus. For the indicated intervention, mixed ANOVAs will be conducted for assessing SEP+ effectiveness. Moderator and mediator effects will be assessed employing Model 1 and Model 4 from the Process add-on for Statistical Package for Social Sciences.