There's two aspects to this - what's right, and what's legal.
In terms of what's right, I think your idea of teaching for free and waiting until you have accomplished a lot before turning pro is best. Obviously the local studio can make rules about who is allowed to teach there, and you are free to go elsewhere if you don't like them.
As far as 'amateurs' being allowed to teach, it depends on what country you live in/represent. Even for the British Open, the rules are NOT the same for everyone, as that comp simply requires that you be considered an amateur by your own country's IDSF organization.
The US organization - USABDA - won't permit amateurs to teach outside of a special program with some srtrict qualifications and limitations, but apparently the rules in much of Europe are far looser. Also in the US, competitions sanctioned by the NDCA rather than USABDA have their own elgibility rules - people allowed to teach by USABDA are not allowed to compete as amateur, though a tiny number of people who place well at the NDCA's national competition are allowed to teach and still compete against foreign IDSF-elgible dancers and domestic amateurs in a special 'world competitor' category.