Conservatives and teaching unions criticised schools for sending home children who disregarded uniform rules.

The Department for Education and Skills has warned heads against ordering pupils out of class to change their clothes.

The ruling emerged when one of Tony Blair's flagship city academies was criticised for sending home pupils after they broke uniform regulations.

Senior staff at a semi-independent state school sponsored by a Christian charity, Trinity Academy, have taken a tough stance on uniforms. They have recently sent home a number of pupils for being incorrectly dressed, including a 12-year-old girl who arrived with false nails and her 15-year-old brother who had spiked hair.

When a local councillor complained on behalf of the children's family, the Department for Education wrote to the academy saying that heads could not "informally" send the children home.

The letter says: "In cases where the school feels the pupil should not be on site as a result of their actions and the situation cannot be resolved immediately, an exclusion is the only way forward.

There is no such thing as an informal exclusion, or a 'time out' or any other way of keeping a child away from school."

But Nigel McQuoid, the director of the Emmanuel Schools Foundation which runs the academy, was reluctant to "blight" the children's record. He said: "Trinity treated this matter as it would an accidental spillage on clothes, damage to a pair of shoes, or some other matter which can be readily resolved, for example, by a trip home or to a hairdresser.

"There was no need for the student to be formally excluded, nor would we wish to do so unless there was a clear refusal to conform to school uniform. The academy would not wish such matters to be recorded as exclusions, given that the term is often associated with children who are badly behaved."

Mick Brookes, the general secretary of the National Association of Head Teachers, said: "It is much better that heads simply tell a pupil to go home and get changed rather than up the stakes by issuing them with a warning and then formally excluding them, all of which will appear on their official record."

John Dunford, the general secretary of the Association of School and College Leaders, said: "Sending a child home to get changed into a proper school uniform is entirely right.

"It is what any sensible parent who has sent their child to school properly dressed would expect if another pupil is not in the correct uniform. It is hard enough for head teachers to enforce school uniform policies as it is."

David Willetts, the Conservative shadow education secretary, said: "Uniform is an area for the professional judgment of head teachers. It is not the job of the DfES to second guess head teachers in matters of school discipline."

Current department guidance says schools should be "considerate and discreetly try to establish why a pupil" is wearing the wrong uniform.