This paper reports the results of the improving thinking skills (ITS-1) intervention study on the thinking skills of low-achieving first graders. The intervention programme consists of 12 lessons, each lasting for 45 min. Lessons offer enriched-discovery learning activities and tasks to be solved through inductive reasoning. We used a quasi-experimental approach, with pre-, immediate post- and delayed post-tests conducted among intervention and control groups. The following groups were formed from a total of 149 first graders on the basis of the thinking skills measure: low-achieving (<-1 SD) intervention group (LowI) (n = 9) and two control groups, comprising both low-achieving (LowC) (n = 18) and well-performing (=-1 SD) groups (WellC) (n = 122). Thinking skills, mathematical skills, listening comprehension skills and reading fluency were measured. The results showed that in the beginning of the study, there were differences in thinking skills, mathematical skills, listening comprehension skills and reading fluency between the LowI and WellC groups, but the LowI group was able to reach the level of their well-performing peers at the end of the intervention in all measures. The discussion focuses on the implications of intervention research, educational practice and responsiveness to intervention.