Your Chromebook has a keyboard, and taking a screenshot with the keyboard can be done in a couple ways.
To capture your entire screen, hit Ctrl + window switch key.
To capture only part of the screen, hit Ctrl + Shift + window switch key, then click and drag your cursor to select the area you'd like to capture.
Note: If you're using an external keyboard, the combinations will be Ctrl + F5 and Ctrl + Shift + F5, respectively.
Submitting a Screenshot to Google Classroom
To take a screenshot on a Chromebook, students hold down the control key and press the window switcher key. To take a partial screenshot, they hold down the control key, the shift key, and the window switcher key.
In Google Classroom, students will see an “ADD” button on the submission page.
They will need to add the saved screenshot from where it is stored.