With the WebRTC Desktop Sharing extension installed in Chrome, you can share your desktop with other Chrome browsers with just a link. Once the extension is installed, you can click on the icon it places in your toolbar to start sharing your screen.
You will need some way to get the link to your students. A url shortener like Goo.gl or Bitly would work. Another option is to use a push system like Pushbullet and push the link as a notification to the class.
Once the class has clicked on the link, they will be able to see everything on your screen. Note, you can also do this with other services, such as Google Hangouts, but that has limits with number of viewers. This extension will support up to 256 viewers at one time.