Look for signs from your child that teasing or bullying is occurring. The obvious signs include visible bruises or injuries. Also, look for changes in your child's attitude, as he may seem anxious, nervous, scared to go to school and perhaps restless.
Find an opportunity to talk openly to your child and find out if teasing or bullying is occurring at school. Make sure the child understands that, if teasing or bullying is happening, it is important to talk to someone about it --- whether it is you or a school authority.
Make sure the problem is still occurring and proceed to talk with school authorities over the bullying or teasing matter. Schools often have penalties or programs that address this matter and can help stop teasing and bullying at school.
Contact the parents of the bully, as this may be the easiest way to resolve the problem. The other parents might be able to help the bully understand why bullying is wrong and help diffuse the situation. However, often the other parents might not acknowledge the issue, and a mediator at the school can intervene to discuss this matter.
Deliberate if the problem has become serious enough to talk to legal authorities. If this is the case, then you should seek legal advice. Often, when the school officials do not acknowledge the problem and signs of bullying still appear, it is time to seek legal advice.