In South Africa, the medium-of-instruction (MOI) debate has continued to demand the attention of educators and academics, particularly after the promulgation of the 1997 multilingual language-in-education (LIE) policy and the introduction of the OBE-NCS curriculum in the schools. Using a survey questionnaire, classroom observations and focused interviews, this study aims at establishing how teachers in selected urban and rural high schools in the Mthatha District understand, interpret and implement MOI policies within their practice. It also seeks to establish reasons for implementing the MOI policies in the ways they do. The study utilizes Phillipson’s English Linguistic Imperialism Theory, Brock-Utne’s Qualification Analysis, and Vygotsky’s Social Constructivism to explain the findings. The main findings of the study are that MOI policies are not implemented uniformly in urban and rural contexts or within each context. Learner linguistic profiles, mismatch between a teacher’s home language (HL) and that of his/her learners, the subject being offered, the need to promote understanding of content, teachers’ understandings, misconceptions and beliefs about the role of language in education: all these were found to be factors which may influence a teacher’s language choice during lesson delivery. Generally, teachers endorse the use of English as a language of learning and teaching (LOLT) at high school, together with the learners’ HL. Although some teachers believe that they use English mainly for teaching, indigenous languages are also used extensively, especially in rural and township schools; code-switching, code-mixing, translation, repetition, and township lingo all make the curriculum more accessible to learners. The anomaly is that assessments are conducted only in English, even in contexts in which teaching has been mainly in code-switching mode. An English-only policy was employed in the following situations: in a desegregated urban school; in a rural high school where there was a mismatch between the teacher’s HL and that of his learners; and also in a rural high school where English was offered as a subject. The most cited reasons for using English only as an LOLT were: school language policy, teachers seeing themselves as language role models, the use of English as a LOLT at tertiary level, and past teacher training experiences. The study concludes that the major factors influencing school language policies in a multilingual country such as South Africa are the school context and the teacher and learner profiles. In addition, teaching and assessing learners in languages with which they are familiar, as well as using interactive teaching strategies, would develop learner proficiency, adaptability and creative qualifications, resulting in an improved quality of education.