Stephen Player, 61, of Manor Road, Spratton, Northamptonshire, walked free from court after Jacqueline Carey, prosecuting, said no further evidence was being offered against him.
The move came during the sixth week of his trial at Ipswich Crown Court and after two other former members of staff changed some of their not guilty pleas to guilty.
Player, who had faced six allegations of cruelty to a person aged under 16, had maintained his innocence since his arrest and charge two years ago, was told by Judge Rupert Overbury that the case against him was at an end.
Outside court, Player said: “I can now enjoy my retirement.”
A second former member of the teaching staff at the now-closed Oakwood School in Stowmarket, Suffolk, Graham Hallett, 66, of Aldcliffe Road, Lancaster, who had faced one charge of child cruelty, was also told that the case against him was being ended with no further evidence being offered.
During the trial, the jury heard allegations that pupils at Oakwood had been subjected to beatings and force feeding of food which staff knew would make them sick.
The allegations spanned the period between 1974 and 1999 and were claimed to have involved around 20 victims.
Gerald West, 70, of Gislingham, Suffolk who was the deputy head, and Michael Watts, 59, of Netley Abbey, Southampton, both changed their not guilty pleas to guilty on two of the charges against them. They are due to be sentenced on Friday.