What are the best free high school supplement course publishers for extra practice in core subjects?

Here are some of the best free publishers and platforms for high school supplement courses / extra practice in core subjects:

1) CK-12

• Best for: Math, science, some ELA/social studies
• Why it’s good: Free digital textbooks, practice problems, simulations, and adaptive exercises
• Strengths: Very strong in algebra, geometry, biology, chemistry, physics
• Format: Online, printable, and flexible for self-paced study

2) Khan Academy

• Best for: Math, science, grammar, economics, history, test prep
• Why it’s good: Clear lessons with lots of practice and mastery-based learning
• Strengths: Excellent for filling gaps and reinforcing school content
• Format: Short videos, quizzes, unit tests, practice sets

3) OpenStax

• Best for: Core subject textbooks, especially science and math
• Why it’s good: High-quality open textbooks written at college level but useful for advanced high school students
• Strengths: Great for AP-style or honors-level enrichment
• Format: Free online textbooks and downloadable PDFs

4) Socratic / Google’s learning tools

• Best for: Quick help with homework questions
• Why it’s good: Helpful for step-by-step support in math and science
• Strengths: Good for checking work and understanding processes
• Note: More of a support tool than a full course publisher

5) School Yourself

• Best for: Math and science basics
• Why it’s good: Interactive lessons that are easy to follow
• Strengths: Good for extra practice and visual learning
• Limitation: Smaller subject range than Khan or CK-12

6) Math Is Fun

• Best for: Math practice and concept review
• Why it’s good: Simple explanations and lots of examples
• Strengths: Useful for middle and high school math review
• Format: Free lessons, worksheets, puzzles, and practice

7) Crash Course

• Best for: History, biology, chemistry, literature, psychology, government
• Why it’s good: Engaging video-based instruction
• Strengths: Great for overview and review
• Limitation: Less practice-heavy than the others

8) Varsity Tutors Free Learning Tools

• Best for: Practice quizzes and skill review
• Why it’s good: Free worksheets, diagnostic tests, and live classes sometimes available
• Strengths: Good for supplementing schoolwork across subjects

9) CommonLit

• Best for: ELA, reading comprehension, writing
• Why it’s good: Free reading passages, questions, and lessons
• Strengths: Excellent for literacy support and textual analysis

10) ReadWriteThink

• Best for: English/language arts
• Why it’s good: Classroom-ready activities, graphic organizers, and writing tools
• Strengths: Strong for writing and reading practice

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Best picks by subject

Math

• Khan Academy
• CK-12
• Math Is Fun

Science

• CK-12
• Khan Academy
• Crash Course for review

ELA / Writing

• CommonLit
• ReadWriteThink
• Khan Academy grammar/writing

History / Social Studies

• Khan Academy
• Crash Course
• OpenStax for deeper reading

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If you want the shortest “best overall” list:

1. Khan Academy
2. CK-12
3. CommonLit
4. OpenStax
5. Crash Course

If you want, I can also make:

• a subject-by-subject ranking
• a list of free printables/worksheets only
• or recommendations tailored to 9th–12th grade, honors/AP, or remedial support.