Various arguments and solutions are proposed in the current discourse on weak English language proficiency in Tanzania. Most authors agree that the standard of reading, writing, speaking of the language is low. Proposed solutions vary. One solution proposed has been to support student language proficiency in form 1 by running a language supportive program. Language Supportive Teaching and Textbooks [LSTT] is one among the recent programs designed and practiced in some schools in Dodoma, Morogoro and Lindi regions of Tanzania. The program with materials available for further experimentation since 2016. This study investigated the enhancement of the proficiency in the language of instructions by using tried LSTT materials to improve English language proficiency among Form 1 students in Temeke district in Dar es Salaam. The study employed action research design; using a quantitative approach to administer a vocabulary and comprehension pre and post –tests, and qualitative research approach aimed at understanding the views of teachers and students through interviews and observation of learning in classrooms. The study was guided by Constructivism Theory (Vygotsky, 1978) focusing on learning by seeing and scaffolding by following teachers and peers, and Bilingual theory of Common Underlying Proficiency model (Cummins, 1980’s). Two public secondary schools were involved in the study, which included a sample size of 160 participants among whom 154 were students, four English teachers and two Head teachers. The researcher selected students from two streams in two schools hence one stream termed as a “treated group” and other group used to compare the results of pre- and post-test. Purposive sampling used to select four teachers. After running eight lessons, the findings revealed that through LSTT materials both teachers and students benefited as classroom interaction improved. Teachers were able to present the lesson well through LSTT approaches and pedagogy, which promoted students participation through exercises and better understanding, as well as improved speaking and reading. Those who were oriented through LSTT material had a higher score in the posttest, which was significant. After the researcher had been teaching through the LSTT materials most of the students in the trial gained confidence and more interest to learn through English language. Their scores in vocabulary and comprehensive test also improved significantly.