The objective of this study was to investigate whether the class scheduling of Nature and Biology classes in blocks results in better learning success for primary school students, and whether this depends on the average student success rate (i.e., student performance categories), age, or prior knowledge. For this study, we have assumed that block scheduling results in better success rates for older lower-performing primary-school students. The research included 773 fifth- to eighth-grade students from 14 Croatian primary schools. The students fell into two groups: one group attending 45-min Nature and Biology lessons twice a week (single-scheduled classes), and another group attending a 90-min lesson once a week (block-scheduled class). To assess the level of student learning success, all students underwent both an initial and final written exam in Nature and/or Biology, specific to each grade. The rmANOVA proved that there was a significant interaction among class scheduling, performance categories, and the initial and final written exam scores of fifth- and seventh-grade students. Such a correlation was not found among the sixth- and eighth-grade students. Our findings further indicate that students achieve better results in block-scheduled classes at the end of primary school education, and that block class scheduling does not necessarily result in improved student achievement, particularly in lower-performing students.