*.tk sounds like a sub-domain suffix used by people who cannot afford maintain their own properly registered website domain.
An organization pretending to be professional and to employ foreign teachers makes me suspicious if they do not seem able to afford to maintain their own domain and must turn to some of the free sub-domains offered in the internet instead.
Hence, when job offers come from such a sub-domain - do not take them too seriously, and still better: do not get involved with them - for your very own sake!

Such job offers made by holders of a sub-domain are mostly scam for obvious reasons!