I remember wen we used to collect the foil caps from milk bottles to generate some cash. It used to take thousands of these for only a small return. However, given that our outlay in effort was negligable and we did get a return it was worthwhile.
The supermarkets are not obliged to give anything away. They simply do it for marketing; and it works. Given that there is no effort on the part of the school other than collecting tokens, which are provided during normal shopping, the outlay is negligable and a return is provided.
There is no such thing as a free lunch so anything you get for no outlay and negligable effort is a bonus.