The aim of this longitudinal study was to gain an understanding of how Swiss student teachers (n = 253), who had passed a competence-based admission test, were assessed by their mentors after the first year of teaching. The results revealed a high correlation between the students' initial cross-curricular competencies (CCC) and their mentors' subsequent assessment. A regression analysis showed that the students' initial CCC had the strongest impact on their subsequent competence development in relation to teaching, whereas cognitively oriented measures, including a cognitive ability test, did not add to the prediction. Qualitative analysis of mentors' assessments revealed different patterns of low performing students' competence development in relation to mentors' support. Students with low self-confidence seemed to particularly benefit from shared reflection and a good mentee-mentor relationship.