Teachers and children of the 1950s and 1960s will remember that the milk was supplied in small bottles containing one third of a pint. Plain drinking straws were provided.
The insertion of the straw into the bottle was challenging. The bottle was almost impossible to open by normal means, which was downward pressure under the palm of the hand followed by a twisting motion. It would open by the 'thumb through the lid' strategy. Use of the thumb was problematic as it had to be discreet given that it was forbidden. The secret was not always well kept as it could be explosively messy.