Intermediate (3-5)

musictheory.net

Free, online, interactive music theory lessons. Covers concepts from beginner to advanced level. There are also tools for ear training and developing fluency in music reading. You can use the material online for free, or purchase iOS apps to do your learning offline.

24/7 Science

A kid's science site from Lawrence Hall of Science at UC Berkley. There several interactive activities, online games, and hands-on experiments available for free. You can also purchase classroom science kits that could be useful for co-op classes. 

Sumdog

Online curriculum and learning games. You can access most of the math content for free, and an inexpensive monthly subscription gets you extra math, plus reading and spelling content. 

42 Electronics

Curriculum and parts kit for learning electronics, robotics, and Python programming. There is one level available now, and 3 more coming out over the next year. Price is pretty reasonable, and the writers appear to be a homeschooling family!

Fat Dogs and Coughing Horses

This oddly named resource comes from the Purdue Univ. College of Veterinary Science. It's a unit study curriculum exploring animal and human health topics. The lesson plans are written for 3rd and 9th grade classrooms, but can be adapted for home use and different level learners. Some lessons require some equipment that may be tough to find, but there are activity books, internet links, and lots of great hands-on ideas. 

Summit Writing Academy

Online writing instruction, including reading and vocabulary elements. You can choose from two levels of membership (one includes teacher grading, the other provides rubrics for parent use.) Or you can purchase 6-week live classes. The price is a bit high, but membership is billed monthly, so trying it out wouldn't be too much of an investment. 

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