What if the Bully is an Adult?
We have to teach our kids to recognize when someone or something makes them uncomfortable, and to talk to us about it. If the adult is a coach or a teacher, you'll need to address the issue with his or her supervisor. If the bullying adult is your ex, I would recommend a two-fold approach: talking with your ex directly about the issue, and encouraging your child to also talk with him or her about it.
Particularly if your relationship with your ex is contentious, he or she may "hear" what is being said more fully if it comes directly from your child. It's hard to blow off a child who calmly and genuinely says, "I'm uncomfortable when you yell at me," or "I don't like it when you tease me."
In addition, if safety is an issue, you should also report your concerns to your lawyer and/or child protective services, depending on how severe the bullying/intimidation has become.
When Should I Get the School Involved?
Any time that a child is being physically threatened or is reluctant to go to school because of bullying, it's time to get school officials involved. At the very minimum, the school can arrange for your child to meet with the guidance counselor to practice some anti-bullying strategies.