Every three years, the Organisation for European Cooperation and Development evaluates the scientific competence of European students. Recent results have demonstrated that scientific competence in Spain is somewhat underdeveloped, which underscores the need to apply educational methodologies that favour the development of said competence. This paper analysed the extent to which the implementation of the Spanish high school diploma of research favoured the development of the Scientific Competence of students. The students had to develop a research project, co-tutored by a secondary education teacher and a University teacher/researcher. Both professionals offered the guidance necessary to develop a project that concerned the resolution of a problem of interest. Audio and video data were collected during the 18 months of the high school programme. These data were transcribed and analysed using a rubric designed ad hoc. This assessment instrument is a relevant point of this work as long as it can be used as evaluating tool in the Scientific Competence assessment of future studies. The results revealed that the students had to make use of their knowledge of the research topic, strengthen this knowledge, substantiate the research using the scientific literature, design and develop the experimental methodology, collect, analyse and present data, and, finally, disclose these data. The results indicated high levels of development in the three sub-competences that were explored, which concluded in a high development of scientific competence.