Science, technology, engineering and mathematics (STEM) outreach programs have been widely studied in recent years considering their possible influence on future STEM career election aiming to counteract the observed decline in enrollment at university. Nonetheless, the presumed effect is not clear due to a lack of comparison with control groups. In order to fill this gap, a quasi-experimental design was adopted to analyze the effect of a STEM experimentation outreach program on 5th and 6th graders. The sample was composed by 453 students, (274 experimental group and 179 control group). The Auzmendi Scale of Attitude towards Mathematics Modified (ASMAm), and the attitude towards school science (ASSci), were used as instruments, and were administered before and after the intervention. The analysis was run with sex, type of school (state and state-funded schools), school environment (rural/urban), and teacher as potential factors. The results show that there is a program effect on the attitude towards mathematics, but not on the attitude towards school science. Regarding the factors, the program effect is associated neither with sex nor with rural/urban schools. However, the program had a more positive effect on the ASSci than on the ASMAm in the state schools, and is mediated by the teacher.