Education Malpractice is the legal term that is used to described the pratice of language schools using unqualified teachers, usually fraudulently issued with TESOl certificates. No one has ever sued a private language school for educational malpratice so far.
From EFL-Law.com... The field of TESOL is a growing social science. Hundreds of thousands of native English speakers teach English language skills to students across the globe. The vast majority of teachers are not qualified to teach English, though the number of instructors undertaking advanced certificate or degree skills is increasing. There is no precedent for the question, "Can a student sue a TESOL teacher for educational malpractice?" but the courts in the U.S, and the commonwealth countries of Australia and the UK give some insights - though differing. The question raises relevant secondary issues that go to an Educational Course providers' duty of care to provide courses that are fit for the purpose and relevant to the student's proposed course of employment.