Once established, they are forever, but I don't know what your specific problem might be, so let me suggest several variables to consider. Your suggestion of contaminant residue in the containers sent out to the schools is possible, but I would guess that routine attention to rinsing during the cleaning process would make this a very low probability variable. Guppies are on the edge of tropical. They are sensitive to temperature change, particularly cooling. A chilling of several degrees over a short period of time may be enough to cause a mortal shock.
Traditional chlorination is relatively easy to manage.
If, however, you have chloramines in your water, chemical treatment is required. Chloramines do NOT dissipate with time and must be neutralized chemically. If local water is a problem, I recommend bottled spring water, at least for starters at the time of delivery. Oxygen requirements go up during times of stress, like relocation. It is desirable to keep the time in transit to a minimum, and it might be a good idea to have an air bubbler in the new container for a period of time to insure good oxygen saturation right after relocation.