Hi Vincent,

I will do what you told me to do save to the extent that your instructioins are inconsistent with the school rules.

I will do what you told me to do to the extent that your instructions are consistent with the school rules.  

 Let's look at simpler examples.

I will accelerate the car to the extent that it reaches the speed-limit.

I will accelerate the car save to the extent that it exceeds the speed-limit.

It seems to me that these, and also your examples, are correct and have the same meaning.

Save is archaic or at least only literary now in the meaning of 'except . .  .' You rarely hear or read it now.

Best wishes, Clive