Developers with an MSDN subscription can now download Windows 10 S and start testing apps on the newest member of the Windows 10 family. Windows 10 S, which only runs apps that are distributed through the Microsoft Store, is going to be pitched at markets where low-cost hardware and simplified system maintenance is important. That's why this initial release is pitched at schools.
The ISOs can be found at the MSDN website. You'll need an active subscription to download them.
Whether Windows 10 S is a success and can take a bite from Google's ascending position with Chrome Books or dies just as Windows RT did will remain to be seen. If Microsoft can get enough developers to port popular applications and make them Windows Store compatible then they have a shot at success.
But the platform will live or die on the support from developers.