What's the best healthy- vegan/vegetarian- cooking class or school in NYC

One more vote for Natural Gourmet Institute..

I use CourseHorse to find cooking classes in NYC and they now have an entire section on veg/veagan classes in the city. http://coursehorse.com/nyc/classes/healthy-cooking

Here are some great ones, thought the site lists many of the ones suggested above, like those at Brooklyn Kitchen.
1. Winter Veg Soups at Home Cooking NY - http://coursehorse.com/nyc/classes/healthy-cooking
2. Intro to Veg Cooking at Bhagavat Life - http://coursehorse.com/nyc/class/introduction-to-vegetarian-cooking
3. Vegetarian Pantry at Purple Kale Kitchenworks - http://coursehorse.com/nyc/class/vegetarian-workshop

To me, healthy and vegan are not necessarily synonymous. Everyone's idea of "healthy" is different, so I'm not sure if these will interest you. I tend to think about flavor first, so the classes I can recommend are all vegan, and all delicious, but not necessarily health food oriented. Plus, they are all taught by friends of mine who are fun, awesome, and most importantly, authorities on vegan cooking!

1. Chickpea & Olive "Burgers & Beers" class at Haven's Kitchen: http://www.chickpeaandolive.com/cooking-classes.html

2. Terry Romero's "Hands on Pie Baking" class at Brooklyn Kitchen:
http://coursehorse.com/nyc/class/vegan-hands-on-pie-making-class

3. Chitra Agrawal's "South Indian Home Cooking" class at Brooklyn Brainery. http://brooklynbrainery.com/courses/south-indian-home-cooking

The Institute of Culinary Education (ICE) also offers some very good classes on vegan/vegetarian cooking

I've heard good things about the Natural Gourmet Institute: http://naturalgourmetinstitute.com/