Mr Bielby, 46, a father of two, said he and Dave Eland, another teacher at the school, timed the dash to overpower the masked man: 'We charged in. As we rushed towards him he reached across to his bag to get the pistol,' he said.
'Dave Eland grabbed his hand and pushed it away from the children. We shouted at the children to get out.
'After that we wrestled him to the ground and got the pistol off him, throwing it to Graham Nellist, the teacher who had been in the class when he first burst in,' he said.
'After that we just kept sitting on him, keeping him pinned to the floor, until the police arrived. He said a few things, but not very much.'
When they arrived at the classroom they noticed blood staining the white blouses of some of the girls, Mr Bielby said.
'We knew then we had to act immediately before even worse things happened.
'Earlier, the man had brandished a pistol but at that point it was in his bag, although he had a knife in his hand,' he said.
Mr Bielby said they initially thought that the pistol might be just a replica wielded by 'some nutter'.
As they peered through the door window, he said, 'we could see how he had made the children line up along the wall, with their faces to it, and he was walking up and down behind them.
'At one point he looked round and pointed the pistol at me so we ducked out of sight and waited for a suitable time to make the move.'
Mr Bielby said: 'When it came to deciding to overpower him we didn't have time to think about it.
'There was nothing special about what we did. Any teacher would have done it if they could have seen what we saw through the window.
'There was no time to think about whether we might be in danger. Our concern was for the children and to do what we could to end the danger they were in as quickly as possible.'
He said he only knew the murdered girl, Nikki Conroy, slightly.
'All I can do is offer my sympathy to her family and to the families of the other children.
'It was an absolutely dreadful experience for everyone, especially the children, and we will all take a very long time to come to terms with it,' he added.