I took a class last winter in Athens. It was sponsered by the Gourmet monthly supplementary by Eleftherotypia. It was held in Greek and it ran once a week for a month with a fairly well known Athenian chef ( who's name escapes me now). It's a demonstration styled class, not hands-on and I would say it's really a beginner course. He demonstrations were more meditteranean recipes as opposed to strictly Greek recipes.
Here is a link to culinary schools in Athens, maybe you could contact them and see if they offer classes: http://chef2chef.net/culinary-institu...
But most of the cooking classes I've heard of are spring/summer sessions in the Islands.
I also hold monthly cooking classes in my home usually for groups of 10-12. I have already given my class for this month, but I could tailor something for you and your travel partners if you like, with whichever Greek recipes you prefer out of my repertoire.