CK-12 Foundation
CK-12 Foundation is a non-profit founded in 2007 by Neeru Khosla and Murugan Pal to provide free K-12 STEM learning materials. It is curriculum-aligned content rather than a curriculum framework: graded lessons and textbooks organised by grade level and mapped to state standards, not an abstract set of competencies. Its core format is the FlexBook, a customisable, web-based textbook that teachers can adapt, remix and reorder. The library leans heavily on mathematics and science, complemented by adaptive practice, interactive simulations (PLIX) and study guides. Some FlexBooks are also offered in Spanish and Hindi.
You access the material by reading FlexBooks online or downloading them (HTML and PDF), and by working through practice items on the platform. There is no open bulk-download corpus or public content API, so ingestion means pulling the per-section HTML or parsing the downloadable books, then splitting on the existing chapter and section structure. Because the content is authored as proper graded textbooks, the boundaries are clean and the reading level is organised by grade, which suits RAG systems and AI tutors that need to sequence explanations by subject and level rather than dump undifferentiated text.
The licence is the crucial caveat. CK-12 content is governed by the CK-12 Curriculum Materials License, which grew out of Creative Commons Attribution-NonCommercial and remains non-commercial in substance. You may read, adapt and share the material with attribution, but you may not use it for any commercial purpose (a narrow carve-out exists for an individual tutor). For anyone building a paid product this is a hard blocker, so treat CK-12 as a reference or a non-commercial-only corpus. It sits alongside the other open education resources we list: where OpenStax offers largely CC BY college texts that are safer commercially, and LibreTexts, MIT OpenCourseWare, OER Commons and MERLOT aggregate mixed licences, CK-12 is the strongest free K-12 option but the most restrictive on commercial reuse.
Related sources
CASE Network (1EdTech)
A public registry of machine-readable learning-standard frameworks from all 50 US states and other issuing agencies, run by 1EdTech in the CASE JSON format. The digitally referenceable spine of what to teach, at which level and in what order, rather than the teaching content itself.
Common Core State Standards
The US Common Core State Standards for English language arts and mathematics, defined grade by grade from kindergarten to grade twelve. A widely adopted curriculum framework, available in machine-readable form through ASN and CASE.
Khan Academy
A free nonprofit learning platform organised as a subject-to-skill knowledge tree across maths, science, economics and the humanities, from primary through early college. Videos, articles and mastery-based exercises, but non-commercial licensing and no public data API.
LibreTexts
One of the largest open textbook platforms, covering higher education with some K-12 material. Hosts interlinked textbooks and reference material across science, mathematics, humanities, and social sciences. Mostly NonCommercial, so commercial use is ruled out.