You should keep the nitrates below 20. Whatever that takes. Usually it means 30% per week, but your own experience will vary with bioload, rate of feedings, etc. Let algae grow on the back and sides for some natural help. It eats waste and feeds certain fish a bit. I love my algae.
You said the tank is located in a school. Is it possible that someone may have tried to clean the glass with Windex or another ammonia based product? Definitely would watch out for stuff like that, my wife almost poisoned my fish one time just spraying the stuff on the glass carelessly.
Yes it would matter. Any air contaminant will make contact with the water and could be absorbed. If you burn scented candles; if you clean with ammonia two rooms away, your fish will know it. If your tank is covered well enough to keep air out, they will suffocate. So will your bacteria.