Here's the relevant NFHS rule.
The sixth item in the list of things that are balks in rule 6-2-4 is:
"Failing to pitch to the batter when the entire non-pivot foot passes behind the perpendicular plane of the back edge of the pitcher's plate, except when feinting or throwing to second base in an attempt to put out a runner."
The case book offers further clarification. If he throws the ball to second in an attempt to retire the runner who is advancing there, it is legal. If he feints the throw to second to drive back the runner who has neither feinted nor attempted an advance, it is a balk.
Notice that under federation rules, it must be an attempt to put out the runner, not merely to drive him back.
So, if the pitcher guesses right and could have a play at second, it's not a balk, even if he fails to throw for whatever reason (e.g., fielder hasn't moved into position).
However, if he guesses wrong, it's a balk.
As an umpire, permit me to add that I'll call the balk as soon as I see that the pitcher is neither stepping and throwing to first (which happens when the non-pivot foot clears the back of the rubber) or delivering a pitch if I haven't already seen/heard/felt the runner make a meaningful move toward second (i.e., more than a typical secondary lead).
So the pitcher has guess right that he's stealing, and he has to guess right that he's stealing on first move.