Although most international students arrive with required language qualifications, many struggle with the linguistic demands of their programmes. This study explored whether the test-preparation industry undermines the qualifications with which students arrive. English proficiency of 153 Chinese student in the UK was tested on the Duolingo English Test and a C-test. Students who attended IELTS-coaching programmes scored lower on both measures compared to students who met entry requirements without such help. Furthermore, the number of attempts to achieve a particular IELTS score was negatively correlated with the other English proficiency scores on arrival. The results confirm that coaching, and to some extent repeated test-taking, boost IELTS scores without generalising to other proficiency measures. The effects were, however, small so that despite the observed inflation, IELTS scores were a reliable predictor of academic success: the rise of one IELTS band resulted in the average grade increase of 9 and 4 points (out of 100) in linguistically more and less demanding disciplines, respectively. The results underscore the important role that language plays in study success, and show that many international students get accepted with levels of English that limit their academic achievement. The test-preparation industry contributes in part to this.