Pupil Katie Gallagher, 15, said she was in a nearby room when teacher Ann Maguire was stabbed.
She said the incident happened in a regular Spanish class with around 30 children witnessing the stabbing. Everyone in her class burst into tears when they were told about the teacher's death.
She said: "I heard her scream. I was in a meeting at the time but I heard it."
Her sister Becky Gallagher, 20, said she left the school in 2010. "I just think it's a shame for all the pupils' future who won't get to be taught by her."
Although a shocking attack, this is a rare attack. There are few other occasions when a teacher was stabbed inside their school by a pupil.
A 15-year-old schoolboy is in police custody tonight, suspected of stabbing to death his teacher.