Ofsted’s plan to put curriculum, rather than exam results, at the heart of its inspections has been broadly welcomed by teachers – but chief inspector Amanda Spielman’s flagship reform has put her on a collision course with the government. The education secretary fears that the move will increase teacher workload, and other key education figures have raised doubts about whether the watchdog will be able to make effective, impartial judgements on the curriculum. The head of Ofsted should prepare for battle, writes William Stewart