Claire Harrington, 26, who recently finished a PGCE at the University of Central England, admits that she cannot escape the classroom - even in her dreams.

“I have always talked in my sleep,” she said. “But it has got to the point where I have woken myself up telling imaginary children off. And I have told my boyfriend to go and wash his hands.

“I come home from school and plan lessons and mark books, so I suppose teaching preys on my mind a lot.”

Ms Harrington even finds herself using silly voices outside school. “I must try not to talk to everyone like they’re six years old,” she said.

Andrew Pinkham, a newly qualified Year 4 teacher from Romford, in Essex, has also made a deliberate effort to stop slipping into teacher mode during out-of-school hours.

“If I’m in town and I see kids misbehaving, I sometimes give them a bit of a look,” the 26-year-old said. “It’s an automatic thing.

“But I’m also more confident talking to people, because the stories I have to tell people are a bit more interesting than other people’s might be.

Last term, one girl said that in Spain they do ‘flamingo dancing’. It makes better stories than accountancy.”

Gemma Horton, 22, who recently finished a PGCE course at Newman college, Birmingham, said: “I’m a changed girl,” she said. “At Christmas, I was playing a game with some family, and I explained the rules v-e-r-y slowly.

“I get really irritated when my friends interrupt me when I’m talking. And when I was having a squabble with my brother last week, I ended up saying, ‘Stop. I’ve had enough. Now go away and think about what you’ve done’.

“I keep using my teacher voice all the time. It’s quite scary. I don’t want to turn into a busybody teacher.”

Beth Dennis, who started last month as a maths teacher at Waseley Hills high, in Worcestershire, agrees that her new job requires heightened self-control. “I don’t like seeing people chewing gum now,” the 32-year-old said. “It makes me think of kids being defiant. But I’d never say anything.”

And what would make her job easier? “I’d like to develop an ability to see out the back of my head. That would be helpful,” she said.

Sara Bubb, of London university’s institute of education, says that this is an inevitable part of becoming a teacher. She admits that she often feels tempted to upbraid passers-by who drop litter in the street.

“It’s very easy to slip into bossy-teacher-mode outside school,” she said.

“But it doesn’t win you any friends. You need to divorce yourself from your teacher role. Too strong an identification is dangerous: if someone says something bad about your teaching, it can hurt the real you.”