This paper investigates the perceptions of teachers from one Sabaot dominated primary school in western Kenya regarding the medium of instruction (MoI) policy in different class levels. While their "ideal" MoI policy bears some resemblance to the official policy which advocates mother tongue (MT) as medium in lower primary and English in upper primary, it also has significant differences because of the teachers' desire to move learners as quickly as possible from MT to Kiswahili and English. The "reality" that learners do not have the proficiency required to comprehend instruction in English, however, means that code-switching between languages is a dominant feature of classroom discourse. Ethnographic data were collected over a seven-month period through lesson observation, semi-structured and focus group interviews, and document analysis. This paper particularly draws on data from Science and Mathematics lessons to provide a lens for observing teachers' actual practice. Implications from the study reveal that policy-makers should make it a priority to address policy goals related, for example, to personnel, methodology and materials, so that teachers can give the learners' first language the attention it deserves. In addition, policy-makers would do well to consider extending the time allocated to this language.