This study examined the use of non-standard English features in 16-year-old students' writing in the UK. Adopting a diachronic approach to the investigation of students' written production, the study sought to identify changes in students' use of non-standard English over the course of a decade, specifically from 2004 to 2014. It involved an analysis of 858 extracts of writing composed under standardised examination conditions by students from across the UK. The study found an increase in the use of non-standard English features over time. This increase affected mainly low-attaining students. While attainment emerged as a factor influencing the use of non-standard English features, gender did not. Other findings reported in the paper include a list of the most commonly used non-standard English features in 2004 and 2014 pointing to the features that should be prioritised in teaching, and indications about their geographical distribution. Some insights into dialect levelling in the UK are also provided. The paper argues that the role of language education should be re-evaluated and re-defined at regular intervals, as students' language needs seem to change over time.