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CK-12 (with Flexi AI tutor)
By CK-12 Foundation (nonprofit). Visit CK-12 (with Flexi AI tutor)
Desk research from the maker's own pages. Not yet tried by us. Checked 2026-10-24.
A free nonprofit library of online textbooks (FlexBooks), practice, simulations and videos in math and science, plus some English and social studies, mostly for grades 1 to 12. Teachers can assign lessons and track progress. Its AI tutor, Flexi, sits on the student home page: a child types or speaks a question such as how to convert pounds to ounces, and Flexi explains, quizzes or points to the matching lesson.
Our take: Useful free study help for middle and high schoolers with an adult checking in; parents who don't want an AI chatbot or AI training on their child's input should look elsewhere.
| Ages it suits | about 6-18 (grades 1-12) |
|---|---|
| What its terms say about age | Users must be 18 (or the local age of consent) or have a parent or guardian agree to the terms for them; under-18s are Child Users whose parent is responsible for their use. The privacy policy treats under-13s (up to 16 by country) as children and seeks parent consent when it learns a child wants to use it. (terms) |
| Price | Free. Free for students, parents and teachers (Oct 2026). |
| Works on | Web |
| Account | The child's own account |
| Ads | None |
| In-app purchases | None |
| The AI in it | Flexi is a generative AI chatbot that answers questions, explains steps and makes practice; the terms warn its answers can be wrong and must be double-checked. |
What's good
- Free nonprofit content across many subjects and grades
- Flexi can explain a step instead of just giving an answer
- Teachers can assign and track work
- No in-app purchases
Watch for
- An open AI chatbot for children; answers can be wrong and kids can use it to get homework answers
- Terms allow CK-12 to use what users type to train its AI
- Voice questions use the microphone
- No independent research on results
Data and privacy
The privacy policy says Flexi questions are not tied to a user's identity and asks users not to type personal details; it may use aggregated or de-identified data to train AI models, and the terms let CK-12 use what users submit to train its AI features. It uses cookies to measure its own ads on other sites. Read the privacy policy.
Does it work?
No independent studies found of CK-12 or Flexi; the maker's pages describe features rather than learning results.
In the Big Index
- On the list Top 50 free learning sites for kids (2026)
- On the list 21 best homeschool resources online
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CENTURY
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A sensible home or school add-on for UK families; families outside the UK curriculum will find less fit.
Where these facts come from
- https://www.ck12.org/student/
- https://help.ck12.org/hc/en-us/articles/51042851054235-CK-12-Terms-of-Use
- https://help.ck12.org/hc/en-us/articles/51042086058267-CK-12-Privacy-Policy
- https://help.ck12.org/hc/en-us/articles/20581985399579-How-to-Enable-Your-Microphone-when-Using-Flexi
Prices, terms and features change. Nobody paid to be listed, nothing here is an affiliate link, and Peabody never rates itself. Where we and the maker disagree, the maker's own pages are the ones to trust.