The Atlanta Journal Constitution reported that 30 students at Finch Elementary School in Atlanta were sent to the hospital (all were treated and released) on Monday, December 3, 2012 for symptoms of carbon monoxide poisoning.
The gas leak was traced back to maintenance work on the school's furnace that was done by "maintenance workers". The AJC's report seems to suggest that the maintenance workers were employees of the school, not Atlanta HVAC contractors.
The school maintenance workers failed to open up a valve on the furnace after doing the work. Which valve was it? The AJC article didn't specify, but my guess is that the valve that was closed caused incomplete combustion, or a vent pipe was closed.