Most efforts to incorporate computational thinking in K-12 education use visual tools (e.g., Scratch) and focus on students in their first grades. Fewer projects investigate the development of computational thinking in students in the last years of school, who usually have not had early formal preparation to acquire these skills. This study provides evidence of the effectiveness of teaching C++ (a low-level programming language) to develop computational thinking in high school students in Chile. By applying a test before and after a voluntary C++ workshop, the results reveal a significant improvement in computational thinking after the workshop. However, we observe that there was a tendency to drop out of the workshop among students with lower levels of initial computational thinking. Besides, tenth-grade students obtained lower final scores than eleventh and twelfth-grade students. These results indicate that teaching a low-level programming language is useful, but it has high entry-barriers.