Headteachers have admitted illegally excluding pupils from school, including one "extreme" case in which children in their final GCSE year were sent home at Christmas and told not to return until their exams.

The government should conduct research to identify the full extent of unlawful exclusions, and recommend measures to prevent a "small proportion" of schools continuing to act in this way, the report says.

David Wolfe, a barrister specialising in education law, told the inquiry that in some cases, academies were attempting to avoid scrutiny of their exclusions by appeal panels, and refusing to hear appeals from parents.

Wolfe said some academies were refusing to comply with official guidance on exclusions. He also claimed some were refusing to admit children with statements.

The report quotes Wolfe as saying that this is the case with a substantial number of schools and is "symptomatic of a pattern of behaviour, rather than being limited to a few bad apples".

Maggie Atkinson, the children's commissioner for England, said: "For the first time schools are on record saying they had illegally excluded pupils. Due to the informal nature of such exclusions it is difficult to know how widespread this practice is but it is worth further examination.

"Our report recognises that exclusion may in rare cases be a necessary last resort. It should happen only if a child is a danger to his or herself or others, or when learning is so disrupted that only exclusion is possible. But all exclusions must be within the law."

The post of children's commissioner was established as an independent champion for young people in England under the Children's Act 2004, the legislation brought in after the Victoria Climbié inquiry.