The Lithuanian education system, along with many others, might address the problem of educational inequality. One of its strategic educational goals is to narrow the achievement gap in terms of socioeconomic status (SES) by ensuring equal opportunities for children from families of different SES backgrounds to optimally develop their competencies during school years. However, the findings suggest that not only did the achievement gap between students not narrow, but it significantly widened between 1995 and 2015 – based on the 20-year Trends in International Mathematics and Science Study (TIMSS). Affordable and high-quality preschool education can be seen as one of the most effective policies to reduce differences in competence between children from different SES backgrounds in the early stages of their development. However, the long-lasting effect of preschool education on academic achievement in elementary school is debated, and these effects might differ depending on the elementary school environment to which children are exposed after preschool education. In this study, using Lithuanian data from the TIMSS 2015, we use 2-level random effects models to examine the links between the length of preschool education of students and their mathematics achievement in fourth grade. We ask the following research questions: 1) Is preschool education associated with students’ mathematics achievement in fourth grade in the TIMSS 2015, after controlling for the SES of students’ families? 2) What are the compositional and interactional effects, with respect to preschool education, on student achievement? The TIMSS 2015 results indicate a significant positive effect of preschool education on mathematics achievement at the student level. However, the composition of the school based on preschool education strengthens the relationship within the school between the student’s SES and achievement. It seems that in Lithuania the problem of educational inequality remains and should be further addressed.