It comes after headmaster Dr Simon Orchard stepped down from his position at the school with immediate effect due to personal reasons.
Mr Jeffery will be working alongside the current assistant headteacher Helen Davis, who will become acting deputy headteacher.
A school spokesman said: “Both Steve and Helen have worked at Sion for a number of years and the school will be in very good hands for the remainder of this term and going into the next academic year.”
Mr Jeffery, who is also mission director and teacher of music at the school, previously worked as assistant headteacher at Worthing High School for a decade.
He started at Sion in January 2015 and, through his dedication to the role of deputy head, has got to know all the children at the school, along with their families, very well, the spokesman said.
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