One of the missions of language education internationally is to enable students to use language in a context-appropriate manner. Against this backdrop, students' ability to linguistically encode formality emerges as an important issue. Using extracts of writing produced by 16-year-old students in 2004 and 2014, this study sought to investigate whether the level of formality in students' writing changed over time. The 858 extracts of writing sampled were composed as part of a high-stakes examination in English Language. The linguistic analysis carried out focused on a number of features serving either as discriminators between spoken and written discourse (e.g. lexical diversity, lexical sophistication), or as markers of informal electronic communication (e.g. abbreviations, omitted stops, non-capitalised sentences). A statistical comparison of the prevalence of these features in the 2004 and 2014 corpora indicated that students' writing underwent informalisation. This trend was found to be stronger among lower attaining students.