Police were called to a primary school in Warwickshire on Tuesday after two schoolboys spiked their teacher's coffee, according to reports in The Sun.
The teacher at Clapham Terrace Community Primary School in Leamington Spa reportedly had her back turned when the 10-year-olds poured a bleach-like substance into her drink.
The Sun said that another pupil warned her of the danger before she drank any of the coffee.
A spokesman for Warwickshire Police told the newspaper that the pupils had not realised the "seriousness or severity" of what they did.
Philip Robbins, the chairman of governors at the school, said police had spoken to the children but that the school and the teacher, Ms Emma Place was not pressing charges.
He added that "nobody knows what the substance was".
A spokesman for Warwickshire Police said advice and guidance was given to teachers and pupils at the school.
A letter about the incident was sent home to some parents at the school, which had recently been given a "good" Ofsted rating.