Mastering a second language, in this case Czech, is crucial for pupils whose first language differs from the language of schooling, so that they can engage more successfully in the educational process. In order to adjust language teaching to pupils' needs, it is necessary to identify which language skills or individual competences set out within the framework of communicative competence should be developed. For this purpose, a new diagnostic test for lower and upper graders of primary schools was designed. Although it is not a high-stakes test, it is essential that its validity, reliability and practicality are ensured, as well as its positive impact on the teaching process, pupils, teachers, schools and society. The present paper introduces the position of pupils with a first language other than Czech in the Czech Republic. It presents a recently developed diagnostic tool and documents the characteristics of the test, such as validity, reliability, impact and practicality.