Formaldehyde levels too high in ONE room

I have had a similar thing happen in one room this year at an elementary school and we think it may be the ceiling tiles. This room (a mini-auditorium) is the only area in the school that has these tiles, and the room otherwise has very little cabinetry. It has failed formaldehyde three times and was over 50 ppb at one point. The concentration was recently in the 30 ppb range so we are hoping it has started to come down now. I retest again tomorrow.  I have also had formaldehyde issues in a small room where I found a plug-in air freshener. I'm not sure if it is a legitimate concern but I made sure to pull those out before retesting and it passed the second time around. In general I think it takes a little longer for formaldehyde to off-gas from products than VOCs. Formaldehyde is usually the thing that doesn't pass for me if everything else does. At least in one or two testing locations. Hope this helps! Good luck! 

Are there any laminates or fabrics in the room not in the other rooms?  Is there any taxidermy specimens, new or different furniture  or different carpet or adhesives?  Any combustion sources nearby?  Was a blank sample sent to the lab along with the other samples to be sure the collection media was not contaminated?  Unfortunately there are no reliable direct reading instruments for low levels of formaldehyde (i.e., <20 ppb).  For about $250 you can get a box of 5 passive samplers (AT571) from assaytech.com with analysis included.  You could place them around the room for 8 or 24 hours, send them back to the lab, and see if you get an area with higher levels than the others to try to track down the source.  Identifying and removing the source would be better than trying to flush.  My understanding of the credit is that all tests have to meet the criteria to get the point(s).