Your first reaction might be disbelief that your well-behaved son or daughter is being accused of bullying.
You can expect them to deny it.
But before you dismiss the thought out of hand, listen to what the school has to say about it. Parents rarely complain to a school at the outset of bullying, there's usually been a history of unhappiness. Sometimes the target has had to take time off school through fear and may be suffering such distress that they need to see a doctor.
You need to take what the school says seriously and work with staff on a solution. There are times when people are unjustly accused of being bullies but a thorough investigation should reveal this.
Sanctions
Sanctions against your child could include a warning, detention, temporary or permanent exclusion (expulsion). A violent, one-off incident harming another pupil could be grounds for expulsion.
If an incident involves violence, text phone abuse or demands for money then the victim and their parents should, and probably will, make a complaint to the police if the bully is over the age of criminal responsibility which is 10 in England and Wales.
You might find it helpful to ask for a copy of the school bullying policy so that you can go through it with your child.