Hey ludwikc 
I’ve actually never heard of Skool until you mentioned them.
I think that before I go into Skool vs Discourse (Its hard not to make a comparison when talking about success) we need to talk about who Skool’s audience is.
Discourse is better suited (but not limited to) communities that focus around a product, support, KBs. In addition it is very searchable such as Discourses’ ‘Advanced filters’ etc. In all, its a perfect place for communities who want an organised community.
Skool on the other hand for me, after having a quick gander at a random site from the discovery list and my first thoughts are that I feel a bit lost. I know why. My confusion is part of its ‘popularity’
Off the top of my head one reason they are popular is because it has a ‘Facebook - style’ look about it. For example, its ‘Write something’ at the top of the page secondly, the way posts / replies are structured.
Skool hasn’t had to lure people in as such due to Facebook unfortunately being much too convenient for users (eg. pre installed app on phones). Whereas, Discourses’ audience is more specialist? is that even the right word?
Another thought occurs that even though Discourse is modern, it may be seen as traditional to people who use Skool or other products.
Over all, I’m not a fan of the pagination, nested replies. Jay’s reply sums it up 