Schools are failing to follow the rules for excluding naughty children, inspectors have warned.
Some schools are illegally "unofficially" suspending pupils, an Ofsted survey of 15 local authorities revealed.
In such cases, children can "slip through the gap" and do not receive any education for long periods.
The survey, which looked at local authorities of different sizes throughout England, found that councils are still having problems identifying and tracking children who are not at school.
None of the authorities surveyed were confident that they knew the whereabouts of every child in their area to fulfil their duty to keep children safe.
Sometimes this was because schools were not aware of their local council's policy for informing them if a child had been suspended or removed from the register.
But in many cases, schools, including academies, were aware of the rules but failed to follow them.
The Children Missing From Education report said: "Even where the local authorities' policies and processes were clear, with an appropriately strong emphasis on safeguarding, if schools chose to disregard them, this could quickly result in children and young people becoming lost to the system.
"Officers in all the authorities surveyed gave examples of schools which had not followed the agreed procedures for excluding children, putting them on part-time timetables or taking them off-roll. This disregard for procedures and legal requirements put the child at risk.
"Half the authorities surveyed were particularly concerned about the risks posed by unofficial exclusions. These are illegal. One local authority officer called these 'back door exclusions - the "Don't come back until we tell you" types'.
