"I enjoyed my time here, I enjoyed the school culture and wanted to be part of that again."
School principal Chris Grinter said Rotorua Boys' was fortunate to have such a high calibre player join the coaching staff.
"To have an old boy like Ruki, who has been playing professional rugby since 2004, is a huge asset to the school and the rugby development programme we've set up.
"Obviously we know Ruki well as a former student and an excellent captain of the 1st XV.
"We know his relationship with the boys and understanding of them will be second to none," Mr Grinter said.
1st XV head coach Ngarimu Simpkins remains in his position.