This article focuses on vocabulary in the context of English‐medium instruction at an international high school in Germany. Longitudinal testing was carried out using the Vocabulary Levels Test (VLT) (Nation, ; Schmitt, Schmitt, & Clapham, ), with 468 participants tested on entry to their first year of secondary school (Grade 6, aged 10–11 years old) and annually thereafter from 2009 to 2015. Learners with the lowest scores on the high frequency (first 2,000 level) words of the VLT in Grade 6 were nonnative speakers, so they were given English as an additional language support. These learners reached mastery of the first 2,000 of the VLT by Grade 9. Nonnative speakers with higher scores on the VLT and native speakers had mastery of the 2,000 level from Grade 6, and all three groups scored similarly at the 3,000 level and academic sections of the test by Grade 10. A corpus‐based lexical analysis of representative textbooks in English as an additional language, maths, and science and learning materials from Grade 6 demonstrates the importance of high‐frequency vocabulary in these texts, and that learners need a large vocabulary to deal with these texts, particularly in maths and science.