The alternative provision census is collected from local authorities on a statutory basis and, as such, does not require data providers to obtain parental or pupil consent to the provision of information.
However, being transparent and providing accessible information to individuals about how personal data will be used is a key element of the Data Protection Act 1998 and the General Data Protection Regulation. The usual way of making this information available is via a privacy notice.
The Department for Education (DfE) provides suggested wording for privacy notices at: https://www.gov.uk/government/publications/data-protection-and-privacy-privacy-notices. The Government provides information on how the DfE collects and shares personal data available at: https://www.gov.uk/guidance/data-protection-how-we-collect-and-share-research-data.