You are correct, it's not. You can turn Windows 10S into full, unlocked Windows 10 Pro in a few seconds, and that's fee for education users.
However, Windows 10S does solve a lot of problems for education users in preventing kids from downloading non-Store apps that may not be easily removable.
I may be wrong, but isn't that basically AppLocker? Why introduce a custom version of your operating system if you could easy merge such a feature into already existing administrative tools (assuming that it isn't, I'm not that familiar with AppLocker)?
Windows 10S is much more flexible and requires no set-up and no maintenance. If it's in the Windows Store, you can install it.
Bear in mind that schools run fleets of PCs and children do not have their own. They get whichever comes off the trolley. The Windows store installs whichever apps they use when they log on.