Know and keep your student's handbook. They should receive one the first day of school, if they don't, ask the office for a copy of your own. Highlight anything in the book that pertains to your student including their conduct, student activities, student sports and other important information.
Use common sense. If the problem can be resolved by talking with the coach (such as if your child feels there is favoritism), then call a meeting with the coach and school principal or athletic director to discuss the issue.
Be discretionary. If the problem is much more serious or may involve law enforcement officials (such as sexual inappropriateness or assault), go straight to the police and have a private meeting with the athletic director, school district superintendent and school principal.
Protect your child. Never dismiss a complaint from your child without first checking out the validity of the accusation. Speak with other parents to see if their children have complained about the coach.