Over 300 teachers have told Sky News that they have been bullied by fellow teachers and colleagues at school.   Niall Paterson is joined by our correspondent Amelia Harper, who's been speaking to some of those teachers about their experiences, including a teacher who faced racist and sexist abuse in the staffroom, and a deputy head forced out for being critical of new school leadership.   Niall also asks Phil Clarke, National Education Union president and teacher, about the use of NDAs in some schools to keep teachers silent - until now.    Warning: This story contains references to suicide along with racist and sexist language.            Producer: Soila Apparicio Editor: Philly Beaumont