Do students under the age of 18, not in college, and not in the physical custody of a parent or guardian have rights under FERPA?

FERPA does not specifically afford minors who are separated from their parents the rights that are afforded to parents and eligible students under the law.  However, schools may use their judgment in determining whether an unaccompanied minor is responsible enough to exercise certain privileges, such as inspecting and reviewing education records and providing consent for disclosure. 34 CFR § 99.5(b).

Parents and Students
K-12 School Officials
Parent and Eligible Student Rights