A jury found Reriti, a former senior teacher at Natone Park, guilty of seven sex charges against the boy, whom she groomed and had sex with once.
Ms Tawhiwhirangi was initially appointed in December as a limited statutory manager to work alongside the board, but take over some of its powers, including employment, finance and health and safety.
Ms Casey said the ministry's "intervention" had now "escalated".
"After the appointment of the limited statutory manager, she identified significant issues at the school that needed to be managed, regarding employment, school culture, self-review and finances. "She also encountered divisions within the board membership that made it difficult to make progress on these issues," Ms Casey said.
There was no set time for the appointment of Ms Tawhiwhirangi and Ms Casey said the school's progress would be reviewed regularly.
Ms Tawhiwhirangi said today her role was to enhance the school and put the pupils' needs first.
"At all schools, when I come in my focus is to work with them to make me redundant as quickly as possible."
She said she'd built a rapport with the majority of staff and the wider school community.
Official concerns about the board's handling of the Reriti situation are found in documents released to NZME under the Official Information Act.
A 2014 internal ministry email notes Reriti's mother is school principal Kiri Smith, while board member Jackson Smith is her grandfather.
But when a police detective met with Mr Smith and board chairwoman Mere Atkins that year, the pair did not disclose Mr Smith's relationship to Reriti, nor did they say he lived at the same property as her.
"When asked, they failed to provide an adequate reason to Detective Warren for not disclosing this information."
Police complained about this, but an email from last year says neither Ms Smith nor Mr Smith were part of Reriti's appointment process, nor part of the board sub-committee "established to manage the situation with the charged staff member".
"Sound processes have been followed in both situations."
Allegations about Reriti's behaviour were taken to police by deputy principal Ria Millan.
The decile one primary school, made up of Maori and Pacific pupils, received a positive Education Review Office report in 2014.