CodeSkool uses websocket to connect to the board. As the website is served on SSL, but the board does not have any SSL, you need to allow mixed content.
https://forum.codeskool.cc/t/websocket-issue/55/2
Scratch uses Scratch Link a separate executable to do that, but that's a wrong choice as per CodeSkool team, so they just ask to allow mixed content.
Also CodeSkool has their own forum, where you can ask any doubts.