Ulster County District Attorney Donald Williams said the singer, who was found with a marijuana cigarette in his ashtray after being pulled over for running a stop sign, spoke at the two Hudson Valley schools in late March.
Garfunkel, 64, would have paid a $100 fine instead.
It was the second time in less than two years that Garfunkel, a Manhattan resident, was arrested for having pot in Ulster County. He paid a fine the first time.
"He was sincere, thoughtful and impressive," Williams said of the Garfunkel speech he attended.
Garfunkel, who with Paul Simon made up the duo Simon & Garfunkel, produced a string of hits in the 1960s, including "The Sound of Silence," "Mrs. Robinson" and "Bridge Over Troubled Water."