i live in Missouri, while im not completely sure about the laws concerning leagal custody...i do know that after a certain age it is no longer the courts decision, it is yours. Also tho, if you are seventeen you can move out, but the first parent is able to take you to court and get you back.
The parents have assumed joint custody.
Joint custody is a court order whereby custody of a child is awarded to both parties. In joint custody both parents are "custodial parents" and neither parent is a non-custodial parents, or in other words the child has two custodial parents.
Missouri don't have a law addressing that. see link
Joint custody of a child can be decided by the parents whereby they share responsibilities for the child. Schedules can be worked out so the child spends time with both parents. Courts can also decide to award parents joint custody in the case of legal disputes and indeed some states have a preference for this in law.
No. Courts routinely award child support in cases where the parents have joint custody.
The one that has the child 51% of the time. see my profile
That should be expressed in the custody orders.
By petitioning the court to give joint custody to the parents. In most state, Joint Legal Custody is the standard. If you mean Joint Physical Custody, with 50/50 Custody, this is more complicated, requiring preparation similar to petitioning for full custody.
If both parents have joint legal custody, both parents must agree on the child going to boarding school. If one parent made the decision with out the input of the other, this would break the custody agreement.
An unmarried mother has custody of her child until the father has established his paternity in court and requested joint custody.