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.
