Nearly £400,000 of work commissioned by a school went to business associates and family of senior leaders, an investigation found.
Essex County Council investigated Wells Park boarding school, for primary-age children with behavioural issues, following whistleblower allegations.
It found payments for unnecessary work and a lack of competitive tendering.
The trust that has since taken over the school, in Chigwell, said robust financial systems were now in place.
The state-funded school provides weekday residential education for about 40 pupils, aged five to 11, who have social, emotional and mental health difficulties, and it also has about 15 non-residential pupils.
