//That’s impossible, could happen in literally any pub, bar or hostelry in the world.//
Indeed it could. But you can lessen the likelihood if you’re a bit choosy.
//In the good old days of free school milk, all children drank it out of the bottle if I remember right.//
You remember wrong. When I was at primary school our daily one third of a pint of milk came in glass bottles with a foil top. Teacher had a device which made a straw-sized hole in the foil top and we were supplied with straws with which to drink the milk. When I went to “big” school the supply was provided in waxed paper cartons (similar in size and shape to a “Jubbly” or “Jungle Juice). These had a small hole, covered with transparent plastic, which could be pierced by the straw that was attached to the carton.
//Furthermore the free milk was to make sure that all children got enough calcium, protein, and fat to help nourish them, thereby improving their health.//
Which of course was, and still is, the responsibility of the child’s parents, not the school or the government. Hence the cessation of free school milk for (IIRC) over sevens by one M. Thatcher when she was Secretary of State for Education, earning her the soubriquet “Milk Snatcher Thatcher.”