There are things to say and things not to say. Telling your child that nothing is worth fighting for sends the wrong message. We don't believe this ourselves, and we sure don't believe it as a county. What the child hears is, "I'm not worth fighting for".
There are a lot of things that are worth fighting for. What we should tell our child is that if you have to stand up to a bully you need to be sure that it is righteous. Ask, "Do you know what righteous means?" Of course, they won't know and that gives you the opportunity to keep the conversation going. Explain righteous in terms he or she can understand.
Next, you go into how teachers are very overloaded with school work and stopping to try and determine who is right and who is wrong in a conflict does not always happen. So, it is easier to just punish everybody than to try and get to the bottom of what is going on. Don't blame, just explain that things are not always perfect.
Lastly, tell your child that if they decide to stand up to the bully and fight them that they are not going to be in trouble with you at home if it was a righteous fight. But they may be in trouble at school. If they are willing to defend themselves and pay the price at school, you will stand behind them.
Our goal in martial arts is to become good enough to be able to walk away from a fight with our head high. It is a lot easier to do, if you know the outcome of the fight even before it starts.