Tangram Supply, a national agency, specialises in using student teachers to cover supply vacancies in primary schools. Its aim, it states, is to help students pay their way. They receive pound;80 for each day they work.
Guidance from Whitehall allows for individuals without qualified teacher status to take on supply work. But regulations also call for relevant alternative experience: "Schools should ensure that supply teachers have the necessary expertise to meet their pupils' needs," says the Department for Education.
"Schools will wish to be clear about the basis on which the supply teacher is being employed: whether as a qualified teacher, as an instructor with special qualifications, or as a foreign-trained supply teacher."
But James Williams, PGCE programme convener at the University of Sussex, questions whether such definitions would include trainee teachers.
"What are the specialist qualifications undergraduates have?" he asks.
"It's fine for them to work as teaching assistants, but if they are given sole responsibility for a class, you're on shaky ground. If the only special qualifications needed are A-levels, why do we bother having a degree-entry profession?"
Trainee teachers, he adds, receive mentoring and guidance in the classroom.
But, if they are working part-time as unsupported supply staff, it may encourage senior management to leave them unsupervised.
PGCE students, who often work five days a week on teaching practice, have little time to take on additional supply work. But the agency will have considerable appeal to undergraduates.
John Howson, TES's teacher recruitment expert, asks: "Will they be willing to take 18-year-olds, if there is no-one else? Is there any quality control?"
Dave Kitt, managing director of Tangram Supply, insists that final-year undergraduates often make more effective teachers than graduates without qualified teaching status.
"In an ideal world, all teachers would have QTS," he says. "But student teachers have a clear understanding of national curriculum objectives and the demands of numeracy and literacy hours. They're young and they're enthusiastic. We allow them to try out different schools and different key stages. If it didn't work, students wouldn't do it and schools wouldn't do it."