I agree he shouldn’t publicly blame them.
But they did break the guidelines (not the law) which had consequences for their ability to do their jobs, so he has every right to be angry (although not in public as that’s not professional). It’s not a case of mind your own business when it impacts your work.
If you come to work hungover/ can’t come to work because you’re hungover don’t you think your boss has the right to be angry?
At work (not teaching) we’ve gone to great lengths to ensure Covid guidelines are followed, make sure everyone can and does distance at work. This is our responsibility as an employer to look after our staff. It is also for business continuity.
If my team went and met up outside work, not social distancing, and not within the guidelines and a number got infected, I’d be furious. All our efforts in the workplace wasted, with potential for significant business impact.