60-year-old Stefan Szczesniuk also entered the plea Tuesday in Chester County Court to possession of an instrument of crime.
Authorities said 15-year-old Raymond Yoder Jr. was killed as he waited for a school bus in West Caln Township in 1986.
Szczesniuk was arrested soon afterward but spent much of the next quarter-century in the state mental hospital. A judge last fall declared him competent to stand trial.
The defendant, who declined to address the victim's relatives, was sentenced to time served plus 12 years of probation. He remains committed to the state hospital.