This problem goes further than nuns. As dairies were guarranteed sales to schools I suspect that the milk they delivered was not the freshest they had available to start with. To this day Ms A refuses to have anything milkier on her cereal than the dyed-white water in the red lid containers.
Despite having to endure this off-milk torture not only at school but at home as well (the old, rancid milk would have to be drunk before the fresher stuff) I somehow survived into adulthood without a fear of the nutritious goodness of a liquid designed to make small cows double in weight every few weeks.