Small bottles [1/3 pint,  c.155ml] of milk were introduced after WW2 in the UK to improve the health of young children in primary [elementary] schools.  A bottle was given to each child at morning break time.

This practice lasted up to the early 1980s when during Margaret Thatcher's premiership it was discontinued.  Her political opponents coined the taunt  "Mrs Thatcher, milk snatcher".  The justification given was that it was not an economic means of providing nutrition, and it was not in any case the role of the education system to do this.

The debate about healthy eating in schools was revived more recently by *Jamie Oliver's crusade to improve school meals, which resulted in major improvements to the quality of food provided in schools.  It is unlikely now, given the increased incidence of lactose intolerance in a more ethnically diverse population, that milk would be considered an appropriate nutritional supplement.

* a well-known TV presenter of food and cooking programmes

She actually did it in 1971 when she was Education Secretary, before she came Prime Minister.

Why? To save money.