Research has been conducted on the detrimental effects of using a foreign language for learning in Tanzania's secondary schools. While most studies recommend the use of a familiar language for instruction, the use of a foreign language in secondary education in Tanzania has been maintained. This has many consequences on the quality of education, and contributes to student dropout. Presenting a study carried out in the semi-rural areas of Dar es Salaam region in Tanzania, this article examines the extent to which language of instruction (LOI) contributes to school dropout. It postulates that the use of a foreign language for teaching and learning in Tanzanian secondary schools hinders the achievement of the United Nations' Sustainable Development Goals (SDGs). The study's findings, drawn from exploratory and qualitative narrative research, indicate that, notwithstanding the presence of several other factors, the use of a foreign language as LOI is a major contributor to student dropout. The use of an unfamiliar language of instruction contributes to students' lack of interest in and disengagement from learning, which ultimately pushes them out of the school system. This is because students usually perform poorly when an unfamiliar language is used as the language of examination and in classroom interaction. The study also reveals that the voices of both teachers and students are not in consonance with what would be expected in an ideal classroom teaching and learning environment. Student dropout from secondary education caused by an inappropriate LOI leads to myriad further problems which, in turn, negatively affect realisation of the SDGs in Tanzania. This article, therefore, concludes that LOI needs to be strongly factored into the development agenda. In order to ensure equitable access to quality secondary education, as required by SDG 4, the Government of Tanzania needs to adopt a language policy that promotes effective teaching and equal access to quality education in secondary schools.