You really need to try and keep tabs on what your child gets up to in school. Try to keep an open dialogue with them, so they feel safe that they can tell you things and you won't go against their wishes.
I think kids get scared that their parents will try to intervene and make things worse with the bully, or that they will be judgmental, or even that their parents have enough worries without them adding another.
It's tough to negotiate those waters, but damage from being bullied can haunt a kid for years. You owe it to them to make sure they don't suffer from it.