Emma Place, a teacher from Clapham Terrace Community Primary School in Leamington Spa in central Warwickshire, reportedly had her back turned when the two 10-year-olds put the substance in her flask.
Another student warned her of the danger before she had a chance to drink the coffee, The Sun reported.
Warwickshire Police said the boys "did not realise the seriousness" of what they had done.
Police said it had been an "isolated incident", which was dealt with internally by the school.
The age of criminal responsibility is 10 in the UK, meaning they could have been arrested and charged with an offence.
Philip Robbins, chairman of the school's governors, said officers spoke to the children but the school and Place were not pressing charges.
He told the newspaper: "Emma dealt with it well. Of course the school is giving her support. The school has dealt with the situation appropriately and we're hoping to move on. It was put in the hands of the police."
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