Children's details from the controversial National Pupil Database (NPD) were passed to the police 21 times, and to the home office 18 times, in the past four years.

It has been revealed that schools were asking parents to submit the country of birth of their kids as part of a census whose results would be stored in the NPD. Fears that this data might be passed to immigration enforcement teams has led to a call for parents to boycott the census. It is not clear how many individuals were involved—one request might involve more than one child.

The NPD figures were obtained by a Freedom of Information request made by Pippa King, whose blog covers privacy and civil liberties issues arising from the use of biometrics in schools. The department for education revealed that: "Since April 2012, the police have submitted 31 requests for information to the National Pupil Database. All were granted, however only 21 resulted in information being supplied."

As for the home office, the DfE said: "Since April 2012, the home office has submitted 20 requests for information to the National Pupil Database. Of these 18 were granted and two were refused as the NPD did not contain the information requested."

The DfE told the Guardian that information had previously been passed to the home office in order "to find missing child asylum seekers or those who abuse immigration control." However, it has now said that there was a new agreement that limited data sharing:

This data has not and will not be shared with the home office or police and there is an agreement in place to this effect. Where the police or home office have clear evidence of illegal activity or fear of harm, limited data including a pupil’s address and school details may be requested.

Jen Persson, from defenddigitalme—a campaign calling for a change of "policy and practice on the personal data of 20 million children in the National Pupil Database"— isn't convinced by the DfE's promises. She said on Thursday that "there was no agreement in place last week. Any new agreement, unless it has a statutory footing, will have no validity and could be changed at any time or the whim of future policy or political will."

The increased sensitivity of NPD data was underlined when the DfE announced last week that it wanted to see advance copies of articles that drew on the pupil database, so as not to be "caught off guard."