Since the introduction of the GCSE and national tests research has drawn attention to the inequality in tiering practices that constrain access to the curriculum and place ceilings on achievement for students in Key Stage 4. The paper builds on earlier research and examines recent data about tiered entry, gender and performance in science at Key stage 3. The paper draws on case study data of science teachers in three schools in Wales to demonstrate how national test results are used to inform decisions about the allocation of students to groups as they enter secondary schooling. The interaction between group and tier allocation effectively locks students on to an assessment pathway from year 7; a pathway which school structures make it almost impossible to break away from. The paper looks at how different teachers respond to this and the consequences for students' access to science and opportunities to learn.