Important: forums or courses, rarely both in one open-source tool
Discourse, Flarum, Apache Answer, and Storyden are community and forum platforms. LearnHouse, ClassroomIO, and CourseLit are course platforms, and LearnHouse also includes community boards. Skool's combination of a feed, leaderboards, classroom, and calendar is closest to pairing a forum with a course platform, or using LearnHouse.
Closest to Skool
LearnHouse is closest, combining courses, community boards, assignments, analytics, and AI in one open-core platform. Discourse is the most mature community platform, with moderation tools, notifications, SEO, and single sign-on, and CourseLit adds course selling and digital downloads.
Payments are often not free
LearnHouse and ClassroomIO are open core: payments, SSO, multi-organization support, and some features are in their paid Enterprise editions, so selling courses may require a license. CourseLit includes integrated payment processing in its AGPL-3.0 code. Check what you need before choosing.
License details
Discourse is GPL-2.0, Flarum is MIT, and Apache Answer is Apache-2.0. Storyden is MPL-2.0. LearnHouse, ClassroomIO, and CourseLit are AGPL-3.0, with LearnHouse and ClassroomIO keeping enterprise features under separate licenses.
Maintenance status
Discourse, Flarum, LearnHouse, ClassroomIO, CourseLit, Apache Answer, and Storyden were all updated within weeks of this update. Storyden is a small, younger project.