I've seen some misunderstandings in this thread. This ruling doesn't create a voucher system. The Supreme Court's ruling sets the bar that a public school has to met in the education of a special education student. If the school does not meet this bar, the guardians of the student can seek reimbursement of private school tuition.
The reason this doesn't apply to all students is because it's specifically about federal legislation that deals with disabled students, IDEA. https://en.m.wikipedia.org/wiki/Individuals_with_Disabilitie...
Edit:See also, FAPE: https://en.m.wikipedia.org/wiki/Free_Appropriate_Public_Educ...
Edit: Here is the decision: https://www.supremecourt.gov/opinions/16pdf/15-827_0pm1.pdf