Being able to use 529 money for elementary or secondary school tuition is a new provision that was included as part of the tax that was enacted in December, 2017. It's still not clear what restrictions may apply. This is what the most applicable section of the statute says:
Any reference in this subsection to the term “qualified higher education expense” shall include a reference to expenses for tuition in connection with enrollment or attendance at an elementary or secondary public, private, or religious school.
I don't know whether or not this includes tuition expenses for attending a school that is located outside of the United States.