If this app was available for Windows Phone, I am 100% sure it would have had more users. It would still not be a huge market, but definitely a bigger one than for a Windows 8 app.

Students start to notice Windows tablets like the Surface more and more so a universal Windows 10 app for Phone and PC would be a good thing now. A lot of users would use it if it was as good as the website and utilised the features of Windows like Windows Ink.

You will most probably not change your mind, but I didn't even know there was a Windows app. You should have made it clearer.

I wish you good luck, however, and I still hope that I might one day see an app for Windows Phone.

I think that, before you ditch the app, you should investigate how many users of Khan Academy are on Windows 8 and Windows 10, and use the app through a browser, then compare that to the number of people that actually download the app.

Given that there are over 400 million installations of Windows 10 on the desktop alone, with a few million more mobile users, I'm sure you'll clearly see that this is not a "small user base" issue but a "poor marketing of the app" issue.

I'm convinced that if you were to treat the Windows Store app as a first class citizen when marketing the apps, putting links and buttons alongside the iOS and Android versions of the app, and communicating that there is indeed a Windows Store app available, your adoption rate for the app would swing upwards.

It seems to me that it has become an unwritten rule in the marketing world that Windows Store apps be an afterthought, even if the development team is working on it at the same rate as apps for the other platforms, but to say that there is a small user base and that's the reason to stop development on the app is a little short-sighted in my opinion.

One of the strongest features of Windows community right now is the ability of its loyal userbase to create apps that fill the gaps that a lack of first-party apps leave. I'm currently using an incredible third-party app for Coursera and that's just one example. If you do leave the store, a willing Windows developer will fill the gap and could do a better job than you can, because they truly believe in the platform, unequivocally.

Please reconsider your position, I'm sure that you will find evidence that this is a poorly-researched decision.

If they already have a site, i can see why they don't need an app.

Also, sometimes windows store will glitch, so the site is better.

Why?

https://developer.microsoft.com/en-us/store/windows-app-data-trends 

I downloaded the app from the app store to use on my tablet, but I found that it was not syncing my progress between when I'm on my laptop and my tablet.

I ended up viewing videos repeatedly and doing the exercises more than once, so that they'd be marked as complete on both platforms.

Also, it's very difficult, if not impossible to access the questions and tips pages or the help center to post anything (question, comment, vote, or answer) on them. 

Finally, it's hard if not impossible to see where someone has responded to a question or answer of mine on the mobile app, because it's hard to find the part of the app that is the equivalent to the website (where your account name is listed on the top right and a little bubble comes up with a number to represent notifications).

This has been true for as long as I've been using KA on either platform and is a major reason that I generally don't use the mobile app on my tablet any more. 

I would use the app all the time if these kinds of problems were fixed so that I could flip between the app and the website depending on where I am and how much time I have - which was my original idea in having both.  

Can KA students work off line on Android device or must everything be on-line

Why couldn't you just leave the old app in the store? As a Windows Phone user one is used to get an app with less functions than the other platforms so the expectations weren't as high. I just discovered KA some weeks ago and so hadn't the possibility to download the app while it was available. It is just a little bit sad to get advertisement for Android and iOS KA-Apps all the time... And no, Shafiq Jetha above wasn't right, no one showed up the last year to make an unofficial version of it. I have heard it is now easier to create Windows apps with Windows Universal apps as a translation of an Android app, so maybe someone wants to give it a try?

The Windows app has not been removed from the Microsoft Store (The formal name Windows Store for Windows 8)