See how productive you are in Chrome with the RescueTime extension.

Source: RescueTime for Chrome and Chrome OS – Chrome Web Store

RescueTime is a Chrome extension that will automatically track how long you use different websites. It categorizes the websites, so you can see how much time you spend in Google Docs vs time on Reddit. This would be helpful for your students to use to see exactly how much time they spend productively. The free plan would be sufficient for students, although they have to sign up with an email address so you’ll need to get parental permission if the student is under the age of 13.

You can set up personal sites and rank them from very unproductive to very productive.