If you are taking lessons through a school where you pay the school and they pay the teacher, tipping is a great idea, because as a rule, the teacher is getting less than half of what you are paying for lessons, and what you are paying is most likely already less than they would get for private lessons. It should be done discreetly. It is not common though, maybe in other societies, and some teachers might be offended. An annual gift might be a more suitable approach, at the end of the year like a bonus. The other thing to be conscious of is breaks for holidays and vacations where the teacher will not get paid, and you should definitely consider paying the amount they would otherwise be earning, so holidays are not gloomy. When I’ve worked for caterers who negotiated a low fee, then gave me a generous tip, like the waiters, I was offended by that, rather than being paid my proper fee.