Is this a balk? Please help!

[steelehead]

Runner on third. The pitcher assumes the windup position. He steps backwards off the pitcher's plate with his non-pivot foot first and proceeds to assume the set position. I was told by a fellow umpire that "in high school, as long as the pitcher does not feint/make a move toward a base (attempt a pickoff) that he can step off the rubber with either foot. But, if he is trying to pickoff a runner - he must step back with his pivot foot.

Which is correct? And can anyone site a rule in the NFHS 2002 rule book?

Thanks!