A one-off workshop can change the way you think about your instrument & its possibilities, WLD says. I've never had a lesson on any of my instruments (some would say it shows), but I imagine -- and clearly I could be wrong -- that it's hard not to become a clone of your teacher. Some of the things I've heard about Noel Hill's concertina schools (and the fierce devotion of some of his pupils to "the system") would tend to confirm this.

A truly great teacher can teach you to attain a higher level than their own -- whether its in school or out.