There is a known issue with Screencastify and MacOS Catalina not recording the whole desktop. You'll have to allow it to record your desktop.

How do I know if I have Catalina?

First, Click on the Apple Icon on the top left of your desktop and choose "About This Mac"


A box will appear. This will tell you your operating system. If it says Mojave on your teacher leased laptop, you're fine. It will look something like this:

However, if it says Catalina, like this:

Catalina users have a few extra steps to make Screencastify work. Follow this link: https://learn.screencastify.com/hc/en-us/articles/360049995693-Failed-to-Start-Recording-on-MacOS-Catalina#:~:text=The%20newest%20version%20of%20MacOS,Start%20Recording%22%20error%20in%20Screencastify.&text=On%20the%20left%2C%20select%20Screen,the%20left%20of%20Google%20Chrome