OSAKA -- For the third time in an unprecedented trial that has lasted more than 21 years, and a quarter-century after the alleged crime, a former nursery school teacher was acquitted Wednesday of murdering a 12-year-old boy in 1974.

It will be difficult for prosecutors now to appeal the decision to the Supreme Court, considering the indictment was first handed down 21 years ago, judicial sources said.

The Osaka High Court on Wednesday upheld a March 1998 ruling by the Kobe District Court that found Etsuko Yamada, 48, not guilty of murdering one of her pupils in March 1974 at Kabutoyama Gakuen, a now-defunct facility for mentally disabled children in Nishinomiya, Hyogo Prefecture. The boy and a 12-year-old girl were found dead in a septic tank on the school grounds March 19, 1974.