Interleaved practice combined with comparison prompts can better foster students' adaptive use of subtraction strategies compared to blocked practice. It has not been previously investigated whether all students benefit equally from these teaching approaches. While interleaving subtraction tasks prompts students' attention to the different task characteristics triggering the use of specific subtraction strategies, blocked practice does not support students in detecting these differences. Thus, low-prior-knowledge students would benefit from interleaving rather than blocking as it guides them through the learning-relevant comparison processes. Because these comparison processes are cognitively demanding, students' need for cognition (NFC) could influence the effectiveness of interleaved practice. The present study investigates the role of students' prior knowledge and NFC for the effectiveness of interleaved and blocked practice. To this end, 236 German third-graders were randomly assigned to either an interleaved or blocked condition. Over 14 lessons, both groups were taught to use four number-based strategies and the written algorithm for solving subtraction problems. The interleaved learners were prompted to compare the strategies, while the blocked learners compared the adaptivity of one strategy for different mathematical tasks. A quadratic growth curve model showed that prior knowledge had a positive influence on students' development of adaptivity in the blocked but not in the interleaved condition. Students' NFC had a positive impact in the interleaved condition, while it had no influence in the blocked condition. However, the effects of prior knowledge and NFC did not differ significantly between the two conditions.