For teachers working for the big companies it's important to realize the separation of company and immigration. Your company does not control your visa. It doesn't have the power to take it away since it isn't the enforcing agency. They may, of course tell the authorities that you are in violation of your visa if you decide to give them the finger as you leave. As in other areas of your life: don't burn your bridges. As a private teacher it may be hard to fulfil your visa requirements. I intend add some links below when I have time. In the meantime, check out this government website for the Ministry of Justice or click on the Immigration Bureau link to go directly to Immigration information.
Links