Our son, DS11, is taking a class with Art of Problem Solving. He loves it! They do move quite fast. It's a 16 week class for Algebra 1.

If you don't want to take online class, they also offer free online learning called "Alcumus"!
http://www.artofproblemsolving.com/Alcumus/Introduction.php

You can choose to follow the textbook and work on a particular topic, and they also provide a detailed progress report. They provide detailed explanation / solution for each problem after you've answered them.