In order to get the Jack of Blades doll from the teacher, you need the rest of the dolls. The student says to show him "all" six dolls, even though there are seven you can collect (besides Jack) and the best fighter of them all (that's right, a doll of you) is actually the one that doesn't count. You can earn the dolls at pubs by beating the score or time limit by about ten (except for coin golf in which I think you need a score of ten) and you will get one for free; with the exception og the Thunder doll which you can buy in Bowerstone North. I'm not really sure what the point of trading all the dolls for Jack is, but I suspect maybe you have to trade Jack for something, who knows?