Both NOVA and ECC are happy for their teachers to gain additional income
working for other companies as long as they don’t steal students and as long as
priority is given to their scheduling.
GEOS does not allow moonlighting and states on its website that moonlighting
is contrary to Japanese Immigration Law. Perhaps this is true, or perhaps not.
The fact of the matter is that most teachers gain valuable income from
moonlighting and no teachers have ever been hauled up by the government because
of it.
In fact, many teachers moonlight as state high school and university
teachers, and the government happily pays them and then happily collects taxes
from income earned on the side.