I am in the process of field testing Zorin 16 Education in our environment and noticed that Veyon is missing. It was in 15.3 Education.
I noticed it missing too, thinking Kolibri had taken its place. Veyon is available for download from here:
On searching there have been dependency issues with Ubuntu 20.04. Not sure if they have been resolved. Current version is 4.7.x
Another point to consider is to replace Firefox with Firefox ESR (Extended Service Release) as only this version can be fully locked down to prevent students accessing sites they shouldn't.