A TEACHER on trial for common assault has said he feared the teenager was high on drugs and about to attack him with a knife.

Clive Harding, 44, the head of PE at Greendown School, Swindon, said the boy's eyes had been rolling round in his head during a maths lesson in which he kept playing with his mobile phone.

Mr Harding told Swindon magistrates he restrained the boy when he ran out of the class.

He said the boy had been walking around class with no expression, that his behaviour was out of character and that he reeked of what Mr Harding suspected was marijuana smoke.

He said when he saw the boy dash to the door, despite being told to stay for detention, he grabbed him by his sweatshirt and dragged him back to his seat fearing he could be a danger to other students.

He dismissed the rest of the class and tried to calm the boy down.

It was then he feared the boy was going to stab him.

"His right arm came up over the table quickly," Mr Harding, of Kingshill Road, Swindon, told the court.

"There was something metallic in his hand.

"I put my hands up in front of me for protection.

"I thought he was going to stab me with something. It might have been a knife or something.

"I was in a panic. I was concerned for myself."

Mr Harding then said the boy placed the mobile phone on the desk in front of him.

Mr Harding said the boy had risen to his feet during the lesson, appearing to sing into the phone, waving his hands around like a hip-hop star.

When challenged to sit down by Mr Harding he said it was as if the boy was looking through him.

"When I reprimanded him he just looked straight through me," he said.

"Looking at his facial expression I thought maybe there was something I had missed about this boy."

Mr Harding said he was not angry so much as anxious for the boy to adhere to his discipline.

He also feared for the safety of other pupils when the boy ran towards a group leaving the class, on May 17 last year.

Under cross-examination by prosecutor Nadeem Aullybocus, Mr Harding, a father-of-two with 14 years' teaching experience, said: "I used the minimum force that I could have."

It was a split-second decision to use force, he said.

He denied swearing at the boy, throwing him into a table or causing him to hit his head on the floor.

Mr Harding said of his style of teaching: "I like to be firm but fair and for pupils to know the boundaries.

He said the boy, now 13, had twice absconded from his lessons, including once just a week before the alleged assault.

After the incident Mr Harding told the headteacher that he thought the boy could be on drugs.

l The case continues.

He added that he had never seen that kind of behaviour from a pupil before and admitted he had been left shaken by the incident.

"I was emotionally shaken. I felt I had the situation under control but then I felt threatened," he said.

The case continues.