The French Culinary Institute (426 Broadway) and Institute for Culinary Education (50 W 23rd) have a good selection for amateurs and professionals. Natural Gourmet offers a pizza class and a bread baking class - it fills up quickly. The bread baking was VERY good.
Le Pain Quotidien on Bleecker is starting to offer bread baking classes.
Slightly dated reviews from Yelp (http://www.yelp.com/search?find_desc=baking+classes&find_loc=New+York%2C+NY)
Scratchbread has a volunteer link (http://www.scratchbread.com/volunteer/)
French Culinary Institute
462 Broadway, New York, NY 10013
Natural Gourmet Institute
48 W 21st St, New York, NY 10010
Institute of Culinary Education
50 W 23rd St, New York, NY 10010