For those teachers sharing their screen and students are reporting the grey boxes covering their presentation, please check the following settings:
When sharing your screen and not showing a video longer than 2 minutes, make sure NOT to check the Optimize for video clip
Also, make sure the 'Share selected app window only' in the Share Screen section in your Zoom App is selected.
If you are showing a video longer than 2 minutes, select Web Optimized and hide the controls