Research aims: Educational research showed that a crucial role in determining student achievement and school effectiveness is played by teachers. At the same time, teachers differ widely in their impact on student performances and previous studies has not been able to identify the sources of this variation. As a consequence, "teacher quality" (more recently called "teacher effectiveness") remains mainly a black box, making it hard to design policies in order to increase it. In our opinion, previous attempt to explain teachers' effectiveness failed due to the fact that they ignore a crucial factor, teachers' relational skills. We consider teachers' relational skills as their repertoire of practices in managing daily issues occurring with colleagues, students and parents. In this study, our aim is twofold. Firstly, we test whether it is possible to foster teachers' relational skills, as defined before and if they actively contribute in determining students' achievement. Secondly, we test whether this improvement may be obtained through light touch training interventions or if it requires expensive investments in long-term teachers' training and their professional development. More precisely, our empirical research tests whether delivering tips to teachers - aimed precisely at better managing on job relationships -- translates for them in higher self-efficacy and in better performances of their students. Intervention: We developed our training program using an innovative methodological approach that combine the potential benefits of a bottom-up developmental framework with those deriving from the reduced training intensity and implementation costs of a light touch intervention. Its building up was based on a Delphi recursive deliberative process that directly involved teachers in the definition of the training contents since the early stages of development, in Italian low secondary schools. The intervention took the form of a brief booklet and six short online videos dealing with several relational issues that teachers are called to face every day at school. In particular, the intervention provides a set of practical tips to better manage the on-job relationships with colleagues, students and parents. The intervention delivery took place in two forms: individual, with only Italian and Maths teachers engaged in the professional development one by one; collegial, with all teachers collectively engaged in the intervention. The RCT was registered (AEARCTR-0002252).