[thecolorblue]

I have been working on my own language arts app for k-12 (https://snowdayacademy.com). In my experience, if it's not fun or rewarding, nobody is going to use it. Most of my users are parents that want their kid to get better grades, or the students want a better studying experience. So, for me its a combination of interesting content, UX, and better feedback than doing a worksheet.

I have been testing it with my two daughters and they make it very clear when its to painful to be useful.

The other thing that worked for me was talking to experts, in my case reading specialists.