This article explores recent developments in linguistic choices in education in Zanzibar and examines the arguments for using local languages of instruction (LoI) as a right. The article's analysis is based on a study of a curriculum change in Zanzibar in which English replaced Kiswahili as the LoI in the last two years of primary school in mathematics and science subjects. The article reviews theory and practices regarding the consequences of LoI for quality education and its implications for human rights in education. The methods included several field visits, observations in classrooms, and interviews in order to address both the reasons behind the curriculum change and its consequences. The conclusion is that expanding the use of English in primary school will reduce the quality of teaching and learning; from the perspective of a rights-based approach, this violates the educational rights of Zanzibari children.