This article discusses issues related to oral assessment of school knowledge of L2-educated students. In particular, it examines benefits and disadvantages of students being tested in their L1 (their dominant language) and in their L2 (their language of instruction). The study draws on the data from 37 high school students studying in a content and language integrated learning (CLIL) setting in Slovakia. They were tested both in their L1 (Slovak) and their L2 (English) on material which they read in English. Their ability to verbally express content knowledge was assessed in terms of linguistic accuracy, fluency, academic format appropriateness and lexical appropriateness. The results from these 37 students were compared with the performance of 35 students who read the same material in their L1 and were tested in L1. The study shows how the choice of either L1 or L2 can to some extent constrain students' ability to express the knowledge they have.