Today, Microsoft announced that it is making Windows 10 S available to MSDN subscribers, who can download an ISO image now. Of course, in order to get it, you'll need a paid subscription, which starts at $539 per year (sorry, but monthly subscribers don't get it).
In a statement provided to Thurrott, a spokesperson said:
“We’re releasing [Windows 10 S] today to developers because we want to be sure they are testing Windows 10 S. We especially want EDU-focused developers to continue to innovate and make apps that teachers and students would use from the Windows Store.”
Windows 10 S was originally announced at Microsoft's May 2 event, which was focused on the educational market. The new SKU of the OS only runs apps that have been downloaded from the Windows Store, but customers will still have the option of a paid upgrade to Windows 10 Pro (it's free until the end of the year).
Microsoft says that beginning next week, Windows 10 S will be made available to Education customers via its Update Assistant application. But if you want to try it out, you're going to have to purchase a Surface Laptop.
At the MicrosoftEDU event in May, we were told that we would be provided with an ISO of Windows 10 S to test and review; however, we haven't received it just yet.