Just rethink those numbers in a class room that is already at somewhere between 1000 and 3000 ppm CO2 to start with.  There are so many studies now of ventilation during the pandemic that showed schools as a rule (not as an exception) have rotten ventilation.  Right here in NYC, we have schools where the only ventilation in the classrooms is one or two windows that for safety reasons only open 3 - 6 inches.  And rooms some don't have windows that open at all.  (What really burned my cork was one of these schools had ordinary A-C units in the windows and told the teachers that this provided ventilation.)

So we need to know a lot about the actual room where this happened.

And it depends on the application.  If they are putting dry ice in water, the temperature of the water will affect the speed of generation.  

Once you remove a person from the high CO2 area, their blood gases should return to normal rather fast.  But they may have sustained damage nonetheless.

The thing you said that was absolutely true and on the mark was:  "I don’t think we will ever get more info on this case."  Schools are as protective of their administrators as big corporations.  Even if this goes to court, it is likely to be settled with a confidentiality agreement on the facts of the case.  I know so much stuff I can't say that I'm bursting at the seams.