Post-transformational neoliberal reforms in the Czech Republic have established a competitive quasi-market, in which -- at the lower-secondary level -- academic-track schools (grammar schools or gymnasia) compete for students in the regular-track schools. These reforms have also brought a rise of shadow education (private supplementary tutoring) provided by individuals and private companies. Even public schools are sometimes providers of shadow education. The study explores gymnasia principals' motivations to provide paid courses to prepare students for entrance examinations to their study programmes and the challenges they experience in doing so. The findings are drawn mainly from semi-structured interviews with principals and other school management members in eleven gymnasia. The study conceptualises such courses as a 'double-edged sword'. First, because it serves as a 'weapon' that academic schools use in the quasi-market competition to attract more (high-achieving) students from regular track schools, and second, because it may have both positive and negative consequences (e.g., for equity in education), depending on the conditions of its provision. The paper contributes to a wider scholarly literature by expanding the understanding of ways schools behave in the competitive educational quasi-markets and by opening a new branch of shadow education research.