Poster is asking about K-12 education for a 529 distribution. I don't find a requirement in Pub 970 for the tuition to be paid to an "eligible institution" to qualify. I only find that tuition must be paid "at an elementary or secondary (k-12) public, private or religious school of the beneficiary’s choosing".
Additional Reference: https://www.irs.gov/newsroom/irs-offers-guidance-on-recent-529-education-savings-plan-changes
