Gordon Gamble, 64, of Filching Road in Eastbourne, East Sussex, has now been jailed for three-and-a-half years.
Gamble was found guilty of acts of gross indecency with a child between March 1967 and March 1970, beginning when he was 15 and the girl was nine.
The Court of Appeal said he should have been been jailed in November.
The judge who imposed a two-year suspended term at London's Snaresbrook Crown Court referred to a number of factors including that the offences were committed when Gamble was a teenager.
He was also acquitted of seven of the original 10 counts, had no previous convictions and had worked as an "exemplary teacher of distinction".
Jailing Gamble, Lord Chief Justice Lord Thomas said judges sentencing in historic cases must guard against any temptation to make decisions because of what had happened in the intervening years.
Referring to the victim, he said: "She has lived with this matter for close on half a century.
"It has clearly had a most devastating effect on her life."
A teacher who abused a young girl nearly 50 years ago has been jailed after senior judges overturned an "unduly lenient" suspended sentence.