Bullying is something that happens in schools far more then it is reported.
The thing to remember is bullying is a "power" behaviour. It is the bully making the victim feel powerless in order to raise themselves up.
As such victims often don't like talking about it, they may feel ashamed or embarrassed. Bullies often target things perceived as flaws in the victim, and talking about it would be talking about their own perceived flaws and weaknesses.
They may also be concerned that talking about it will make it worse, that they will be then bullied for being a tattle-tale or a snitch. That they will be perceived as being weak for seeking out help.
They may also fear losing privileges if they do report it. For example that telling adults about online bullying may cause them to lose access to online activities.
The important thing is to keep in mind that bullying might happen and go completely unreported. Bullying is a problem, but it is not one that can't be dealt with, however kids often need help in doing so and it is up to the adults to watch for signs and keep open and non-judgemental lines of communication.
And remember, bullying is a power and control behaviour. Solving it involves giving power back to the kid, or rather reminding them that they still have it and helping them use it. The best solutions are when they have options and make the decisions on their own. This doesn't mean don't help, but it does mean that they should be involved in the solution and not have more power and control taken from them in an attempt to solve it.