In this study, the effects of some student (e.g., gender, bullying, etc.) and school variables (e.g., emphasis on academic achievement, clarity of teaching, etc.) on the TIMSS 2015 science and mathematics achievement of eighth grade students in Türkiye were examined by controlling for the socioeconomic status of the students at the student and school level. The analyses were performed using the multilevel modelling method and the HLM8 package program. The findings show that school variables account for 34% of the variability in the TIMSS 2015 science achievement of eight grade students, while student variables account for 66%. Similar to this, school variables account for 35% of the variability in these students' mathematics achievement and student variables for 65% of it. The socioeconomic status of the school at the school level and students' confidence in learning the lesson at the student level are the two variables that have the strongest effects on students' achievement in science and mathematics. According to the results, other variables that have a significant effect on students' achievement in both science and mathematics at the school level are the clarity of teaching, the emphasis on academic achievement, and the school bullying level. Furthermore, school discipline problems have an effect on students' mathematics achievement. However, school resources and teacher qualifications do not have a significant effect on student achievement. Home educational resources and bullying among students are two important variables that effect how well students do in science and mathematics. The effect of gender and value learning the lesson on science achievement was significant, whereas the effect on mathematics achievement was not. The effect of like learning lesson on student achievement is significant only for mathematics.