Alright, just for starters, in Somervile, MA there is Openair Circus which offers classes:
OPENAIR Circus: Classes
And Simply Circus in Newton, MA:
Simply Circus Community
Of course, Circus Smirkus is in Vermont.
This may also prove a bit helpful:
Search Results.
The American Youth Circus site, in general, could offer guidance.
I'm still searching for other stuff and asking around. Hope to come up with more.