The Chromebook app is only available for centrally managed devices and if the school has purchased an upgrade for ChromeOS devices. More information can be found on the Chrome Education and Enterprise Help site. 

You can't install the Chromebook app on private Chromebooks due to Google policies.

Install the kiosk application on Chromebooks

The Google administrator (for the managed devices) installs the applications using the Google Admin panel. It will then be pushed automatically to all devices.

To find the Exam.net application, an extension id is needed. Contact us at info@exam.net, and we will send the id and instructions for installation as soon as possible (we usually respond quickly during CET business hours).