Background: Key Word Signing (KWS) is one system that can be used to support the communication needs of children with Down syndrome (DS) who attend mainstream school. The success of KWS in schools is mediated by staff experiences and perceptions of KWS. The current study is one of the first to explore KWS use in mainstream schools. Aims: To explore the perceptions and experiences of teachers and special needs assistants (SNAs) with reported varying levels of experience, using KWS with children with DS and their peers in the first year of mainstream primary school. Secondly, we aimed to explore how those perceptions and experiences evolved throughout the year. Methods & Procedures: Five mainstream schools were recruited where children with DS attended. A speech and language therapy (SLT) researcher taught 25 key word signs to the relevant class in each school at four time points across the year. Five teachers and eight SNAs from participating schools engaged in a series of semi-structured interviews to explore their perceptions/experiences of using KWS ("Lámh") over the course of the year. Longitudinal data were analysed using inductive qualitative content analysis. Outcomes & Results: Four categories were generated from the data: Challenges, Facilitators, Benefits, and Incorporation into the whole school environment. Findings suggest that the supported introduction of KWS in school by an SLT researcher can assist inclusive educational practices; facilitate children's comprehension and expression; reduce frustration in children with DS; and facilitate interaction with peers. Results also indicate an increasingly positive attitude among teachers and SNAs towards KWS as the year progressed. Teachers reported the absence of a pedagogical framework as an obstacle to the integration of KWS into the curriculum, while SNAs highlighted inconsistent access to funded entry-level training. Conclusions & Implications: Overall, using KWS was a positive experience that supported communication in children with DS in mainstream school. However, future training should embed knowledge on key facilitators and consider how challenges can be addressed to maximize the potential of all children with communication needs attending mainstream primary school.