This is a right guaranteed by law - all children are guaranteed to a Free and Appropriate Public Education. In some cases, a private school is more suitable to a student. A school district is not responsible for the cost of a unilateral parental placement at a private institution, however, unless the program is necessary to fulfill the student's right to a free appropriate public education and public placement is inadequate.
The primary vehicle for delivery of a FAPE to students with disabilities is the IEP.