The partnership model 'School Adoption' was developed in Norway as a part of teacher education and in the meantime, is being implemented in several European countries as an internship concept. The core element is the so-called 'adoption week', during which student teachers teach all the lessons at a school, while the teachers attend a professional development course. University teacher educators work intensively together with student teachers and teachers. The student teachers who participate in the School Adoption experience their future professional field under increasingly real conditions and also in its entire complexity. How the complex internship is processed cognitively by student teachers, has not yet been examined. This study at the German University of Flensburg, is designed to evaluate student teachers' concerns and stress experiences, and determine which factors are relevant for being able to deal with the high level of complexity. Content analyses show that student teachers' biggest concerns are classroom management and recognition as a teacher, as well as an increase in the workload. The findings also indicate that internship settings with a high level of complexity, can be perceived as a positive challenge in the context of sufficiently structured preparation and support.