We compared two tools that have been used to capture the linguistic heterogeneity and achievement of students in England: the exposure-based distinction between English as an additional language (EAL) and monolingual learners, and the 2017--2018 five level teacher rating of proficiency in English (from "New to English" to "Fluent"). Based on a nationally representative sample of 140,000 students aged 5 to 16 years, we assessed the explanatory power of the proficiency in English rating in relation to educational achievement and compared it directly to EAL status. Our results demonstrate that proficiency in English is a significantly better predictor of student achievement than EAL status and that it accounts for up to six times more variance than other student background variables (ethnicity, gender and socio-economic disadvantage) combined. Proficiency in English was particularly (but certainly not solely) predictive for student performance in subjects such as English and reading vis-à-vis mathematics. Our findings are clear in demonstrating the value of a proficiency in English rating for assessing linguistic heterogeneity and student achievement, in contrast to the exposure-based EAL measure. We recommend the (re)introduction of proficiency in English ratings to monitor and support student progress and discuss the value of classroom-based language assessments.