Bernard Haunch, 77, of London, denied 16 counts including rape and other serious sexual offences that allegedly took place between 1975 and 1983.
He taught at the now-defunct Fonthill Preparatory School in Coombe Hill Road, East Grinstead, West Sussex.
The jury at Lewes Crown Court failed to reach verdicts on 15 of the 16 counts.
Mr Haunch, of Sutton, faced charges of sexually assaulting a girl and a boy in the 1970s and 1980s.
He pleaded guilty to indecent assault against the boy between July 1981 and July 1982, and indecency with the same child between 2 July 1981 and 1 September 1983.
Judge Michael Lawson directed jurors to return a not guilty verdict on a charge of indecent assault against the boy and discharged the jury after it could not reach a decision on the 15 other counts.
The jury had been deliberating for four days.
The date of the 10-day retrial is to be fixed and Mr Haunch was released on conditional bail.
This article originally appeared on bbc