Edward Timpson MP, the Parliamentary Under Secretary of State for Children and Families, has written to East Sussex County Council children's services to raise his concerns.
He wrote: "For a school so comprehensively to fail to protect a vulnerable teenager from a manipulative adult, who has been placed by the school in a position of trust, is an abrogation of leadership and responsibility which had terrible consequences for this child and the family."
He added: "The way that school leaders dismissed warnings made by young people about an improper relationship, and the failure to provide any effective support to the child, is inexcusable."
More top news
The first female paramotorist has made it across the English Channel - flying the route of migratory swans from Russia to Kent
Monday night's weather for the west of the region
Monday night's weather for the east of the region