It appears from your question you are the payor of child support. If your child drops out of high school after she turns 18, your child support obligation may be subject to termination. Whether this would happen automatically depends on the language of your order and the notice to withhold sent to your employer, if applicable. To be safe, a petition to terminate support stating the appropriate factual basis should be filed as soon as possible after the occurence of the event. You should...
You do not specify whether you are married to her mother or not. This is important to know as it will lead to further questions if you are not. Under either circumstance, if there have never been any legal proceedings related to custody, you will need to petition the court for relief. You may or may not get temporary relief, but you will be ordered to mediate the custody/visitation issues before the court will make a ruling. When you say illegal, that suggests in my mind you are...