I'd like to hear from anyone who has dealt with this or has ideas. From their reputation you'd think they would be bullet proof, indestructible little critters. But, quite to the contrary, they are sensitive to changes. 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.