Student Chromebooks are restricted to installing a pre-selected number of apps and extensions. The full list of whitelisted apps & extensions can be found at the link below:
To install a Chrome app or extension:
1. Load the app or extension's page in the Google Play Store (follow the link above or browse for the exact app/extension.
2. With either the page loaded, or the exact extension appearing in the search results, click the "Add to Chrome" button
3. Click "Add extension" (or "Add app" if it's a Chrome App)
4. Most (though not all) extensions will be added to the menubar and display a notice that the install went OK
5. Should you see the error below, you are attempting to install an app/extension that is not on the whitelist, double-check the link
6. If you need to remove the extension, right-click its menubar entry and click "Remove from Chrome..."