This study investigated the role of using unplugged computing activities on developing computational thinking (CT) skills of 6th-grade students. The unplugged computing classroom activities were based on the Bebras challenge, an international contest that aims to promote CT and informatics among school students of all ages. Participants of the study were fifty-three 6th-grade students from two public middle schools in Istanbul. The unplugged computing activities involved the tasks with three different difficulty levels covering the CT processes found to be common in CT definitions in the literature. To evaluate students' CT skills, two equivalent tests were constructed from Bebras tasks considering the same parameters (difficulty levels and CT processes). The results showed that students' post-test scores were significantly higher than their pre-test scores. There were not any significant differences between students' scores in terms of gender, and there was no interaction effect between students' CT scores and their gender.