Windows 10 S should be live on MSDN by the time you read this. You will need a (paid, expensive) MSDN subscription to access this software.
“We’re releasing [Windows 10 S] today to developers because we want to be sure they are testing Windows 10 S,” a Microsoft statement notes. “We especially want EDU-focused developers to continue to innovate and make apps that teachers and students would use from the Windows Store.”
Next week, Microsoft will make Windows 10 S available to its Education customers alongside an Update Assistant tool. This will help these customers “make informed buying decisions regarding Windows 10 Pro EDU or Windows 10 S for their schools,” Microsoft says.
But it’s not for you. So Microsoft has not answered my call to open up Windows 10 S to Windows Insiders for external testing.