I'm not very familiar with KhanAcademy besides the beginnings of it where it was just a youtube channel. What are they doing now to warrant half a mil lines of code?
At least in mathematics, after each video, they have a fairly sophisticated set of tests that are quick to pass if you’ve understood the concept, or that adapt in difficulty to your level of understanding and attempt to fill in any gaps. It’s quite impressive. Kids can definitely use it to catch up or work ahead on their own if they are interested.