The purpose of this paper is to present field research, which aims at assessing proficiency in Greek of children attending minority primary schools in Western Thrace. The study is part of a larger educational reform project entitled "Education of the Muslim Minority children" funded by the Ministry of Education and the European Union. Language proficiency was assessed on the basis of a rating scale provided by the Common European Framework of Reference for Languages. This scale, designed primarily for adults, was adapted for young learners, and served as the point of departure for designing and developing three tests, one for every two grades of the six-grade primary school. Each test consists of four sections: listening, reading, writing and grammar. Speaking was assessed through a structured interview of increasing difficulty, common for all test takers. In this paper, the main stages of test development are presented. Emphasis is given on describing test specifications with respect to features of tasks (such as instructions, text and task types etc.) and the rating procedures. Finally, some key results are presented, which show that the language variety of the majority of children fossilizes at a fairly low level of language proficiency.