IMHO kids who disrespect and disobey instructors (and otherwise lack self-discipline) don't deserve to be trained. I reckon instructors are fully within their rights (and are obliged under duty of care) to exclude disrespectful/disobedient students from training.

Learning a martial art is a privilege, not a right. Techniques are potentially dangerous and can hurt people. If a student isn't willing to show the proper level of maturity to handle such a responsibility then they don't deserve to learn it.

JMHO.