Board Thread:Spoilers!/@comment-3038174-20130429210540/@comment-24674051-20141002132207

Also, we're going on about who Snow's mentor is. For the longest time, Snow was weilding Robin's Bow (which she got from Rumple). Somewhere before 3x2 (Lost Girl), she must have switched bows, as she is having trouble hitting a target, and David reaches over and helps her adjust her aim.

So, based on that, I would say that Snow was taught magically initially, and David is the one that helped her shoot without magic.