In addition, some parents' health insurance policies may include provisions that allow the child to remain on the plan until they reach a certain age or until they graduate from high school, even if the child is over the age of 18. If your parents' health insurance policy includes such a provision, you may be able to remain on the plan until you graduate from high school, even if you are over 18 years old.
You should check the laws in your state and the specific terms of your parents' health insurance policy to determine whether or not your parents can legally take you off their insurance if you are 18 years old and still in high school.