Most qualified teachers here now are non-native speakers. The native speakers where either grapdfatherered (for the full teaching licence), got qualified, left the country, or found some other way to get a work permit. 5% of the teachers in my school are native speakers (all qualified except 1), and most work in the upper high school. 20 years ago it was close to 100%.
Plenty of trash teachers have come and gone...mostly native speakers employed through an agency.
Note this is a private school and we really do not have the problems that plague the OP's school and many other schools here. Certainly there are few discipline problems and my work submission is close to 100% and on time.
There are better schools out there...