Personal data identifying hundreds of children's names, addresses and schools has been "misplaced", prompting an inquiry at a hospital trust.
A missing document contains the details of all Year 7 pupils in two Greater Manchester secondary schools.
Bolton NHS Foundation Trust has launched an internal investigation after the school nursing team lost the list with details of 425 children.
The Information Commissioners' Office has been informed of the data breach.
It took place as the school nursing service transferred records of children moving from primary to secondary school, the Local Democracy Reporting Service said.
'Commitment to transparency'
To make sure school nursing care continues efficiently, the trust receives a document listing pupils in each class.
Medical director Francis Andrews said managers were taking the data breach "very seriously" and apologised for the error.
He said: "Regretfully, during the transfer of records this year, a list of 425 pupil names, dates of birth, address and schools has been misplaced. The records do not include any health information.
"However, as part of our commitment to transparency and the need to protect confidentiality, we have reported the data breach to the information commissioner and launched an internal investigation.
"We have also written to the parents and guardians of the young people named on the list to explain what has happened."
A spokesman for the information commissioner said the office would assess the case.