In the last twenty years fewer than twenty teachers have been struck off for incompetence in the whole of the UK. That is despite the fact that education experts put the figure of bad and failing teachers closer to 15,000. With new research showing that the standard of your teacher is the most important factor to a child's educational attainment in a school, we ask why so many bad teachers are being allowed to teach. Talking to parents, head teachers and school watchdogs, Panorama reveals the shocking effects that bad teachers are having on our children and uncovers the loophole that could allow struck-off teachers back into our classrooms.