A person or place for bike classes for teen with weak motor skills

I can't recommend someone specifically but I have two suggestions. First, strider bikes for little kids have the pedals and cranks removed so one learns balance and steering first with feet on the ground and off at will, then pedaling, once one is balanced with feet off the ground easily. This could be replicated by purchasing a beater/too small bike first, then moving to the correctly sized bike with pedals. Second, hold the child not the bike. During the phase when anyone is learning to ride, our first go to is to hold the bike. But if you place two fingers of each hand (or perhaps with an older child the palm of each hand) by the armpit/upper rib on the side of the new rider and hold them up from there (obviously easier with a smaller lighter person), then they naturally 'retreat' from the pressure of your hands and in this way learn the core stability necessary to balance. This works better than seeking to control the bike.

Try BORP Adaptive Cycling in Berkeley