Starting in 2018, qualified educational expenses for 529 plans will include up to $10,000 a year in tuition and expenses for primary and secondary school expenses (public, private, or religious). Previously, you could only use it towards qualified college expenses. There were also some related changes to ABLE accounts for individuals with special needs.
Put simply, you can now pay for up to $10k a year of private K-12 school through a 529 plan. If this impacts you, you may consider making a 529 contribution now before December 31st, 2017 as you are allowed annual contributions of $14,000 per person ($28,000 per couple) while still avoiding gift taxes. You would then be able to make contributions in both 2017 and 2018.
Using the 529 as a just-in-time pass-through. Around 30 states offer a in-state tax benefit on 529 plans. If you are paying for a private school anyway, you may be able to save some money by simply using the 529 as a pass-through account. Contribute to 529, grab the tax benefit, and then immediately withdraw (starting in 2018) to pay for K-12 tuition. Some states like Montana and Wisconsin specifically disallow this in-and-out practice, but most do not (although they could start).