zoom causing kernel_task to spike - cpu usage off the chart

Appears to be a compatibility issue with the Zoom app running on macOS Big Sur. Have you contacted the Zoom developers yet to see if they have a solution or updated version of their app specifically for you version of macOS?

Ref: How to contact an app developer - Apple Support

That's strange. I have taught all the school year online using Zoom, with an external display attached to my MBP, with no issues.

Maybe there is something else. Is Zoom fully up to date? Do you have some cleaners, or antivirus?

Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.