i don't know of any organizations, but i've used the "undersized bike method" (outlined at the link; scroll down) to teach an 8 year old to ride on two wheels within the course of 3 hours. 

http://sheldonbrown.com/teachride.html


On Thu, Apr 24, 2014 at 2:45 PM, sheldon mains <sheldon@spokesconnect.org> wrote:
Just got a call (found our website) from someone looking for an organization in Grand Rapids Michigan that offers adult learn to ride classes--
Anyone have any suggestions?
..............
Sheldon Mains
SPOKES Bike Walk Connect
Minneapolis, MN

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