I can’t answer this question as it applies in all jurisdictions, but I can tell you what the law is and how it applies in the jurisdiction where I practice divorce and family law (Utah).
This is a common question, and a good one. It arises with most parents when their children reach the age of 18 or are close to it, usually because the parent paying child support wants to know when his or her child support obligations terminate.
The law in Utah regarding the interplay between a child turning 18 (which is the age of majority in Utah) and child support is governed by Utah Code § 78B-12-219. It provides, in pertinent part:
78B-12-219. Adjustment when child becomes emancipated.
(1) When a child becomes 18 years old or graduates from high school during the child’s normal and expected year of graduation, whichever occurs later, or if the child dies, marries, becomes a member of the armed forces of the United States, or is emancipated in accordance with Title 80, Chapter 7, Emancipation, the base child support award is automatically adjusted to the base combined child support obligation for the remaining number of children due child support, shown in the table that was used to establish the most recent order, using the incomes of the parties as specified in that order or the worksheets, unless otherwise provided in the child support order.
So did you catch that? Child support doesn’t necessarily end just because a child turned 18 years of age. If a turns 18 but the child’s normal and expected year of high school graduation is still a few months away, child support continues until the child’s normal and expected year of high school graduation. This also means that if a child has graduated from high school before turning 18, child support continues until the child turns 18.
Your specific question, however, was “How do custody laws work if the child is eighteen, but in high school?” The answer to that question is that a divorce court does not have the power to order an adult child of divorced parents to reside with or exercise visitation or parent-time with one or both of his/her parents. So even if the child support obligation continues after a child turns 18, that does not mean that an 18-year-old child (in other words a legal adult) is obligated to comply with child custody or parent-time orders anymore. Once a child turns 18, he or she is now no longer subject to the guardianship of his/her parents, which is why a court cannot compel an 18-year-old adult child of divorced parents to spend time with either or both parents. It has no jurisdiction to do so.
Utah Family Law, LC | divorceutah.com | 801-466-9277
Eric Johnson’s answer to How do custody laws work if the child is eighteen, but in high school? – Quora