Not a Catholic but an Episcopalian here. My younger brother used to pass out as an acolyte during the service. The air didn't circulate too great around the altar and he usually hadn't eaten anything for breakfast. Low blood sugar. But, that's probably not the same thing as what's happening here.
Most churches now use 'choke-less' incense. It isn't supposed to clog you up or make you sneeze or any thing else. BUT, it still bothers me!! Every EAster our processional goes like this....crucifer with the cross, acolytes with the torches/candles, then the thurifer, carrying and swinging the 'dreaded' thurible with the incense in it burning away. THen comes the poor choir, sopranos first, right behind the smoke. Try singing, with the smoke pouring out in front of you, the heated air of a full church and then add in the smell of all those lilies!!! I'm always right behind the incense and I do get all 'choked' up, don't care what the box says. And as the Altar Guild Directress, I know what the box says. But, as far as the kids passing out, I wouldn't think it would be from the incense unless they are using a huge amount of it and the kids are in the direct path...it does rise pretty quickly. I would think that the kids are either tired, or hungry or their circulation is just messed up from kneeling/standing...or a combination of them.
I would speak to the 'powers that be'. YOu don't want a whole group of kids traumatized and worrying about passing out in school.