Educational Pioneer. Born to a father who was a teacher, Haanstra followed in his father's footsteps, following an evening course to become a teacher while obtaining practical experience working at his father's school. After finishing his studies, Haanstra worked as a teacher in IJlst, subsequently followed by Strijen and Leiden. In Leiden, Haanstra came in contact with the ideas of Fröbel, inspiring him to think about the development of young children. In 1868, Haanstra was appointed Head Teacher of the Leiden Teacher Training College, additionally teaching at the kindergarten connected to the college. In 1870, he was additionally appointed teacher at the National Teacher Training College in Haarlem. He quit working at the kindergarten, but remained teaching in Leiden as Head Teacher of the Teacher Training College and the Military House of Detention. In 1882, Haanstra was appointed Headmaster of the Leiden Teacher Training College. He remained in this position until his retirement in 1922. During his time as headmaster, Haanstra developed his own kindergarten teaching method, known as the Haanstra method or the Leiden method. This method emphasised self-discovery learning, natural development, and hands-on experiences, advocating for individualised, child-centric education, connecting children to nature, and positioning teachers as facilitators in the learning process. The method aimed to foster holistic development and a love for learning in young children. While the method showed Haanstra was ahead of his time, his contemporaries were critical of Haanstra's school, especially since it largely relied on Haanstra's own songs and stories and mainly used teachers educated by Haanstra himself. Despite this, the school enjoyed a great reputation, further stimulated by Haanstra's many publications. One of Haanstra's students was Mata Hari, who attended the school from 1892 to 1895, but was dismissed after it was rumoured she had an affair with Haanstra. In 1912, Haanstra was decorated as Knight in the Order of Orange-Nassau.