Everyone is scared of installing applications on Windows either because they break the system or because you might be accidentally installing malware. In either case, the end result is countless wasted hours backing data up, reinstalling the operating sytem and all the applications.
An AppStore wont fix this.
For a Windows appstore to work, they need to guarantee that installing software wont ever break the system.
This is something that everyone knows, and hangs over every decision and discussion of every operating system for PC's, phones, the web, and everything in between, but it is rarely directly stated. It is a fundamental problem that everyone must resolve.
And you know what would be awesome? If things called "national educational technology plans" would point out issues like this that are immense impediments to schools using computer technology to its fullest. It isn't just recalcitrant teachers screwing up ed tech, it is half-assed junk technology, too.