The importance of the role of language in teacher education programmes and in children's learning is crucial. This study focuses on the use of English as the language of learning and teaching and its impact on the language development of English second language (ESL) student teachers and ESL learners. Against the background of major theories in second language (L2) acquisition and learning, this topic is contextualized within the South African education system. An empirical inquiry was carried out in which portfolios (evidence of practical teaching including lesson plans and learners' work) submitted by final year student teachers enrolled at a large distance teaching university for the Advanced Certificate in Education: Inclusive Education were scrutinised. A comparison of teacher and learner written errors was made. Based on the findings, a questionnaire was designed to determine the extent of the impact of teachers' limited English proficiency on learners' English proficiency. The findings of the questionnaire responses are presented. Recommendations are made on how student teachers can improve their teaching practice to ensure quality ESL teacher input and ESL learner performance.