As long as you have your current position (employment in my friends case or university enrollment in my case) you can extend the visa on those grounds, basically making you business a "side-business".
After you quit your job or leave university, your visa stays valid until the end of the term.
When the visa expires, you will have to apply for a Business Manager (経営者) visa, because you are not an employee (授業員) at your company, but an executive (役員).
