The impact of a school based assessment system on the implementation of an applications/modelling based curriculum at the senior secondary level is examined. In formal examination conditions, context‐separable and standard applications were common. In unsupervised, extended time conditions, tasks with low scaffolding allowed students to generate their own modelling solutions, whereas tasks high in scaffolding had the modelling structure already supplied by the task setter. The persistence of high task scaffolding in assessment tasks throughout the course, and the concerns of teachers about the reliability and ownership of assessment in unsupervised conditions, appeared to be restricting opportunities for students in some schools to demonstrate their modelling competencies.