The majority of secondary school pupils in Tanzania are multilingual, speaking at least three languages which include: ethnic community language (there are currently more than 120 of them), (Ki)swahili, the national and first official language and, at varying levels of competency, English which is accorded the status of second official language. A very small number of pupils have access to French, since the language is taught only in a few schools as an optional subject. In public primary schools, pupils are generally bilingual, speaking their ethnic community language and (Ki) swahili. Due to their bilingual/multilingual knowledge, they are expected to activate each of the languages in their repertoire according to the situation of communication and to its level of formality as well as to the purpose of communication, the participants and their various characteristics, identity factors, etc. However, in certain institutional settings, the activation of the language repertoire is determined by the norms established by the schools. This paper is intended to: firstly, describe the formation of the bi/multilingual repertoire of Tanzanian primary and secondary school pupils and the nature of their language practices outside of school settings; secondly, indicate how the language practices are modified by the school and, thirdly, explain the ideological and/or pedagogical origins of the linguistic norms set by schools. The conclusion will attempt to explain the impact of the linguistic norms on the perception that the pupils have about the different languages in contact.