The employer doesn't have to pay any money to get a visa for their employees.
I've sponsored a work visa myself once, it was pretty straightforward.
Technically when the terms of employment change the foreigner is meant to go to immigration to renew their status, but of course no one does that, they wait to get another job where the boss will agree to sponsor their visa.
Unless that English school paid for the plane ticket and a bunch of training it's no loss to them.
The English schools I worked at all gave me a whole day of paid training lol, not much spilled milk to cry about
