Police are investigating the crime, which took place at St Joseph’s Roman Catholic Primary School in Whitburn, and have issued an appeal for witnesses to come forward.
Uniformed officers have been carrying out inquiries in a bid to snare the thieves.
It is understood a slab of milk cartons had been left outside the school.
The snatch and grab could not have been more ironically timed – taking place on Thursday, hours after the end of World School Milk Day, which aimed to educate youngsters about the benefits of milk while encouraging them to drink it.
Thousands of schools across more than 40 countries took part in the celebrations, including First Class Nursery in Livingston Village, which was attended by local MP Graeme Morrice, who swapped his parliamentary duties for those of honorary milk monitor.
Mr Morrice said he was “shocked” by the theft, which he described as “disgusting”.
“It’s appalling and I cannot believe someone would do that just after World School Milk Day,” he said.
“It’s pretty callous of the thieves to snatch the milk from the mouths of babes and almost as bad as Mrs Thatcher when she was education secretary in the 1970s.”
The former prime minister earned the monicker ‘Margaret Thatcher Milk Snatcher’ after famously removing free school milk for over sevens in a bid to cut costs to meet election pledges on tax.
Councillor Andrew Miller, executive councillor for education at West Lothian Council, said: “It is astonishing that anyone would take milk from the school.
“Luckily on this occasion we were able to contact the supplier and get new delivery.
“We would like anyone with information to contact the police.”
A police spokesman said: “Officers are investigating whether this has been a childish prank or something more malicious and anyone who remembers seeing anything suspicious in or around the school is asked to contact police.
“Similarly, anyone with information that can assist with our inquiries is also asked to come forward.”
Anyone with any information can contact Lothian and Borders Police on 0131-311 3131, or Crimestoppers anonymously on 0800 555 111.
Last month, the News told how around £300 worth of milk had been reported stolen from a number of Lothian’s schools in the last year.
In 2010, thieves stole £650 worth of milk from schools – more than 40 separate cases.