In 2011, students with special educational needs (SEN) were part of the sample of the National Assessment Study (IQB-Ländervergleich 2011) in primary schools. The study examines if the achievement tests in German and Mathematics are also suited to assess SEN students' proficiencies. To evaluate the structural validity of the achievement tests for SEN students in special education and regular schools, missing patterns, the fit of the Rasch model, differential item functioning, and correlations of the test scores were analyzed. Results show that the achievement tests captured the same proficiencies in similar ways for SEN students compared to students without SEN. However, comparability of the achievement tests in Mathematics was reduced for students in special education schools.