Born and raised in the Port Richmond, Philadelphia, Siemiontkowski played for Northeast Catholic High School.
In 1968 he was part of the varsity squad suspended by coach Jack Friel for returning late to school after breakfast.
The story made national headlines as the North junior varsity team then defeated Bishop McDevitt in the quarter finals of the Catholic League playoffs.
Siemiontkowski graduated in 1968 with the highest single season scoring average in North Catholic school history at 22.6 PPG (later equalled by Joe Schoen in 1977).
As a junior, he scored a career-high 31 points (also grabbing 15 rebounds) in an overtime win against Western Kentucky in the semifinals of the 1971 NCAA tournament.