In the face of the global sustainability crisis, schools and teachers are called on to transform education and implement educational innovations to strengthen sustainability education. One of these innovations is 'open schooling': educational practices that involve students in identifying, exploring, and tackling real-world sustainability problems in the school environment and the local community. Such -- often externally driven -- innovations can disrupt teachers' habitual ways of thinking and acting and create what John Dewey calls 'problematic situations'. This article reports on explorative case studies in Belgian and Swedish schools that establish open schooling practices through LORET - Locally Relevant Teaching. It presents and discusses six different types of problematic situations that were observed as implementing LORET disturbed established teaching habits. Teachers experienced difficulties (1) to plan lessons starting from a sustainability challenge, (2) to take the students along in an authentic quest for solutions, (3) to plan lessons not knowing what the students' input will be, (4) to collaborate among teachers, (5) to define and coordinate the roles of teachers and non-school partners, and (6) to create lesson plans and teaching materials from scratch.