Ask the child if she/he is being bullied by other kids. Watch for different behavior from the child. Talk to the child's teachers. Then sit down with the child and talk to him/her about bullying and what it is.
Also find out what the school's bullying policy is. There should be a form available to be filled out and turned in each time an incident happens.
Only about 1/3 of bullying cases are actually reported. Kids don't tell adults for many reasons: