It's a judgment call from the umpire.....so the answer is yes and no....NFHS rule 6-2-4b requires a pitcher to step directly toward a base....if not then its a balk....
The exception is that a pitcher can throw to the F3 in an attempt to pick off a base runner....but, in the umpires judgment, that first baseman has to be in the "proximity" of the base..... if he is no balk.... if he isnt then it is a balk.
In your case if F3 is 6-7 feet from the bag, that umpire must have felt that was not close enough for an attempt to put out the runner....so he called the balk.....
In most cases, and I'm talking practical application here, if the first baseman is moving towards the bag at the time of the throw, many umpires would not call a balk here.....but that is an NCAA interpretation and not strict NFHS....
hope this helps