There are two issues here: the volunteerism issue, and an idiot instructor.
If done correctly, an instructor program can be useful to the student and the school, and be legal. The school I attend has an instructor program. Students wanting to become instructors may assist instructors in classes as often as they want to, and they are needed. This allows the students to see if instructing is what they really want to do, and allows the head instructor to see if the students have instructor potential.
Minor students who proceed in the instructor program may test locally to become trainee instructors. After a few years of experience and meeting national requirements, they may test nationally to become junior certified instructors. Once they are at least age 16, they may be hired as employees or they may teach occasionally in return for free tuition. Minor trainees are always under the direct supervision of an adult instructor. The head instructor talks to all parents regularly and minor students with disciplinary or school problems are counseled and, if they do not improve, they are either suspended until they improve, or they are dropped from training or the instructor program.