Lisa Gocoul picked up her six-year-old son, Vinnie up outside Hawthorne Primary School on Elmham Road, Cantley, yesterday, to find him drinking from a carton as he left the school building.
She said his school milk carton rattled, leaving her concerned.
She opened up the carton and found there was ice inside it. But she then became concerned about the smell of the carton, which she described as smelling of fish.
Then she looked at the date on the carton - which she said was marked with the words: "16 Mar"
This was five days after March 16, she said, on March 21.
Lisa, of Coterel Crescent, Cantley, said: "It was the first time Vinnie has had milk for ages.
"He gets milk for free. It has not made him poorly, but I think it is disgusting. I spoke to another parent, and they agreed is smelled of fish..
"He had it at the end of the day, and I was concerned that he was going to be ill because of this. Why are the children being given milk that is out of date?"
She said she had contacted the school and been told that the school had spoken to its supplier over the issue.
The Free Press contacted Hawthorpe Primary School, but the school has declined to comment.
The school is a Doncaster local education authority school, but Doncaster Council has also declined to comment.