Parents have taken children out of Lakenheath Primary School after two bottles of milk were contaminated with cleaning products.
Some parents have complained they were not told of the incident until a week afterwards and were shocked the police had not questioned staff after more than a week.
The vigilance of staff meant nobody was given the milk from the half-used bottles. It was revealed at a meeting with parents on Tuesday that one bottle was in the breakfast club fridge and the other in the staff room, which has a key-code door lock.
Suffolk Police believe the substance was added in the school between Friday, October 6, and Monday, October 9, when it was discovered by staff.
A police spokesman said: “The school has swiftly introduced additional security measures including adding locks to classroom fridges and disposing of all unused and non-sealed produce daily.
“No further incidents have been reported at the school or others in the area.”
Head Sally Esom said: “Additional safety measures have swiftly been introduced at the school to ensure that food and drink is only consumed from sealed packaging.
“We are supporting Suffolk Police in their investigation.”
Ann Frase, who took her son out of school after the meeting, said: “What surprises me is the lack of communication between the school and parents.
“We have the right to decide for ourselves whether it’s safe for our children to go to school.
“They’re saying ‘we’re throwing away opened food every day’ but then saying ‘it’s safe for your kids to go to school’. How do they know that?”
She and another mother said police admitted at the meeting that they had not then questioned staff and a woman said she was the cleaner on duty that Friday but had not been interviewed.
A Suffolk Police spokesman said: “Staff at the school are being spoken to by police this week as part of our investigation into the incident.”
He said police were treating it as an attempt to administer a noxious substance but were still awaiting test results.
A spokeswoman for Suffolk County Council said: “It is a parent’s responsibility to ensure their child is in full-time education. Suffolk County Council will continue to support the school and the police in their investigation.”
Anyone with information should contact Suffolk Police on 101 quoting 37/66067/17.