Are you the child's guardian? I'm assuming not as you ask about 'grans rights'.
If not then the school should not talk to you unless you are one of the listed contacts for the child. If not then it would be a data breech.
If you are concerned you should talk to the child's parents, however if they aren't concerned and are supportive of the schools methods then there is nothing you can do.
Also, what is this punsihment? Is it extra time spent on practicing rather than free play (which many schools I know would do)? If so, then that is not a punishment, it is extra support for the child.