This study discuss the implementation of Baitul Arqam Advance as an example of In-Service Training for developing principals' competences and islamic school leadership in Muhammadiyah schools that are in line with Muhammadiyah goals. One of the objectives of holding BA Advance is to improve the quality of Islamic leadership and the competence of school principals. There are two competency targets for school principals that MPK PDM Sleman wants to develop through Baitul Arqam Advance, namely religious competence (including competencies related to Muhammadiyah) and managerial competence. These two competencies are seen as most needed by school principals in Islamic schools, especially Muhammadiyah schools. Because the principal is a pillar of da'wah in Muhammadiyah schools, a role model for the family and society, must be able to lead and manage schools towards superior, advanced, and developing schools. Muhammadiyah as an Islamic organization that has established more than three thousand formal schools in Indonesia since the early 20th century until now, has an important question: how do principals in Muhammadiyah schools develop their competences and leadership through Baitul Arqam Advance? Data collection was carried out through in-depth interviews with Muhammadiyah Regional Cadre Training Council in Sleman Regency (MPK PDM Sleman) as the organizer of Baitul Arqam Advance and principals in Muhammadiyah schools as training participants. The results indicate that Baitul Arqam Advance is quite good at developing Islamic school leadership, but still inadequate in increasing principal competences. This study poses questions concerning the Baitul Arqam Advance curriculum, the focus of competencies developed in Baitul Arqam Advance, as well as its weaknesses and strengths. This study also implies that there is a need to develop principal competences materials and supervise how the training is followed up.