Nowhere else in the world are children assigned to a secondary school level as early as in the Netherlands: at the age of 12. International research indicates that delaying tracking has a positive effect on equal opportunities and academic trajectories. It remains unclear how this affects students socio-emotional functioning, and what role certain underlying mechanisms play, such as the quality of differentiation and the objectivity of determination (level assignment for the upper years). We investigate these effects and mechanisms, and together with schools with heterogeneous and extended first-year classes, we develop an approach for differentiation and determination.