This study examined the influence of informal language learning on academic writing among secondary school learners at Mumbwa Secondary School. Reports from ECZ and other scholars highlighted increasing use of informal language in academic writing, prompting the need to investigate its causes. The study was guided by three objectives: to identify teaching methods used by English teachers to promote formal vocabulary, to determine factors influencing learners’ use of informal language, and to explore remedial measures employed by teachers.A case study design was adopted, focusing on Kawanga Day Secondary School. The study involved 20 purposively selected learners from Grades 10 to 12. Data were collected through interviews, focus group discussions, and document analysis of learners’ exercise books, and were analyzed thematicallyFindings revealed that teachers mainly used structural and contextual methods, such as reading comprehension, to teach vocabulary. Learners’ use of informal language was largely influenced by social media, peer communication, and popular music. Additionally, remedial lessons were conducted once a week, which was found to be insufficient to fully address the problem.The study recommends that learners should actively develop their understanding of formal vocabulary and distinguish it from informal usage. It also recommends that teachers adopt more effective and consistent methodologies for teaching vocabulary and writing skills.