Before you throw antibiotics at the flock, is the discharge clear or actually snotty? If clear, then could be environmental.
Sneezing, in absence of other symptoms, can be dust, ammonia, pollen or chemical exposure such as new caulk or plywood.
I would NOT treat them with antibiotics for sneezing. If they really had something like CRD, it wouldn't cure it anyway- the birds would remain carriers and you'd be fighting recurrences in the flock periodically.
One symptomatic chicken should be removed from the flock and observed for more symptoms without giving antibiotics. Also, observe the rest of the flock that is left in the coop for symptoms. I have one hen who is 5 1/2 and has sneezed daily for 4 years. It's just her being extra sensitive.
If it has breathing problems, you might consider a little Vicks Vaporub under the wings. I don't guarantee that it isn't toxic, only that I have used it with good results on chickens, and the baby rub on chicks