Web-based Curriculum

Mathigon

An interactive math learning tool for jr. high and high school levels. This free resource guides students in discovering and applying mathmatical concepts through interactive, online content. There is also a story element to the presentation that makes it very engaging. There doesn't appear to be much in the way of practice exercises, so you may want to suppliment if that's important to you. But it's a great way to get students engaged in math learning.

Math Goodies

This site has quite a few free math learning resources. There are interactive lessons, printable worksheets and a worksheet generator, a few simple flash-based games, some webquests on fun math topics, and more. It's a no-frills website, that's not terribly interesting visually, but there are plenty of useful tools to explore.

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.

Zoom In

This is a great free resource for history learning in jr. high and high school levels. The interactive lessons in American history help students explore primary documents and write essays using supporting details. 

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. 

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. 

Biology Corner

A website with lesson plans, worksheets, labs, and activities on biology topics. Materials appear to be for jr. high and high school levels. It may take some digging to find exactly what you want, but everything is free.

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