Phil Willis MP is a former headteacher at the school

The spokeswoman said there was an "innocent, but complicated" explanation for the incident, which happened on 19 April.

She said: "Plainly, our teachers do not use guns as a means of maintaining discipline in our schools.

"A teacher was having difficulty with a group of pupils and asked for assistance, as is normal procedure.

"She went to get help from a colleague, who had been just about to start a physics lesson on sound and light, where a starter pistol is used to show how light travels faster than sound.

"He came into the class with the pistol inside his clothes, believing it would be hidden.

"However, one of the pupils noticed it and said: 'He's got a gun'.

Full investigation

"The teacher then took it out to show to the class and explain what it was.

"However, this did cause considerable distress to some pupils. The pistol was not fired at any stage.

"We are obviously concerned at the distress caused to the pupils and we will carry out a full investigation of the incident to see what can be learnt."

Liberal Democrat shadow education secretary Phil Willis, a former headteacher at the school, said: "We should be concentrating on core problems of student behaviour and teacher shortages which all too often are closely linked."