Depends on one's point of view, unless the food is inedible, I'd rather provide the restaurant feedback after I've eaten (and payed my bill.) I tend to be less concerned about my personal outcome and more concerned with the long term. So, to me, if the teacher issues are things that could impact other kids, then a polite phone call/discussion with the principal about your concerns about their employee would seem an appropriate way to provide feedback. But I would want to make sure I was doing it for positive future outcomes and not to be punitive.
