The measures are being adopted by an increasing number of schools who believe they engage students and improve teaching. But the NASUWT argues the trend risks damaging teachers' professional status.
"We are not opposed to the concept of student voice but this is a step too far," said Chris Keates, the general secretary. "Teachers should not be sucked into thinking it's acceptable."
The union had received a flurry of complaints suggesting teachers were alarmed by the growing popularity of such practices, she said.
"These strategies strike at the heart of what constitutes an appropriate pupil-teacher relationship," Ms Keates said. "At this rate, it will only be a matter of time before someone suggests pupils should be trained as Ofsted inspectors."
Hundreds of schools have taught pupils to interview candidates for vacant teaching posts or to observe and give feedback on lessons. At George Mitchell school in Leyton, east London, pupils, referred to as "lead consultants" are involved in a Making Learning Better scheme where they give staff tips on how to improve classroom displays, teaching styles and discipline.
Peter Kent, the head of Lawrence Sheriff school in Rugby, which runs a similar scheme, said: "Although it's more daunting than a visit from the inspectors, I found pupils to be fair and constructive."
The practice is backed by Ofsted and heads are required to give evidence that they have taken pupils' views into account on school self-evaluation forms.
The National Union of Teachers said it was important heads sought consent from staff so that the schemes did not seem threatening.
School Councils UK, a charity that promotes pupil participation, recommends teaching pupils how to rate teachers objectively, rather than leaving it to opinion. "It can be incredibly useful to have a different pair of eyes in the classroom," said a spokesman, Asher Jacobsberg.