- Edited
There are teachers of all sorts ... some are parents (or others) teaching kids, and those parents (or others) may be very good teachers ... without having received certificates etc. And those parents or others may be both very good piano players, that had lessons ... and also very good at teaching their skills to others. And there are the professional teachers ... with certificates etc.
But ... of course, we have to support the 'professional' teachers as well. And we know there are all sorts. In any group or population ... you encounter various sorts.
As for piano ... it is a musical instrument as we know. And it and music and learning and etc means different things to different people.
As for teaching and learning ... there will be case by case situations. And it all takes time to find out what the situation is ... such as a student isn't putting in enough time or effort ... and why ... eg. other social/recreation/leisure activities, or a high workload at school, or not interested, or is one sort that doesn't learn at some average rate due to what reason there is, or due to another teacher not teaching 'correctly' (can be subjective) ... etc. Or a combination of mix of those conditions. There can be grey areas.