That dang team at Sparkbox, always doing amazing stuff. They wrote about it here and even used CodePen at time to help out:

Everyone started at various levels of technical knowledge at the beginning of the day, with some girls feeling quite intimidated by writing code themselves. And when we talked about the theory and concepts of HTML and CSS (for a very short time as these are teenagers!), some girls seemed even more shaky they would catch on. However, once we used CodePen to demonstrate the real-time output of markup changes, many people made the connection almost instantly and began marking up their own content with confidence.

Teaching with CodePen has been quite the thing lately. We love seeing photos of it in action, and have collected some on our Flickr account, like this, this, and this. Professors use it too!