Action research (AR) is increasingly located in schools. Partnership and participation are key aspects of quality AR and this thesis considers how to bring about authentic co-researcher participation leading to positive change through AR in schools. The first paper is a thematic synthesis of literature exploring how action researchers report on projects where adults and young people participate as co-researchers. The papers were selected and reviewed using a PRISMA framework and AR quality criteria. Papers judged high quality provided a show and tell of both process and participation. Review findings showed the importance of building trust and egalitarian relationships, and supporting people (with skill and resources) to enable co-researcher authentic participation. A model is suggested to support participation, considering the elements within context which support engagement. Clear exposition of co-researcher evaluation and reflections on participatory experience provide clarity for action researchers seeking to work with young people and adult co- researcher groups within future AR. Paper two describes action research with seven school staff in a high school in the North of England. This paper explores co-researcher experiences of participating in action research using cooperative inquiry, in a project focused around improving primary to high school transition practice. Four inquiry cycles were followed by further evaluative workshops. Evaluation identified positive outcomes within themes of academic, organisational and social transition systems. This paper is focussed on critical reflection of the factors that facilitated partnership, mutual engagement and positive collaboration towards agreed outcomes. Learning from this cooperative inquiry supports the utility of participatory and collaborative approaches in development of understanding, systems and practice within schools. The final paper focuses on disseminating this research, reflecting on how AR can be used in school to bring about positive change. The paper explores evidence-based practice and practice-based evidence within educational psychology and considers how educational psychologists, acting as change agents, can promote and facilitate AR as a positive approach to school systems change.