Pupils from two schools in the same Hampshire town required hospital treatment after vaping.
The headteachers of the schools in Romsey issued a warning to parents after several children experienced accelerated heart rates far exceeding normal levels, as well as confusion.
One pupil even suffered from fits and a loss of consciousness.
The letter, sent jointly by The Mountbatten School and The Romsey School, said the incidents are being investigated.
The illnesses are believed to have been caused by vapes that had been tampered with and had questionable contents.
The academy schools – which have both been rated good by Ofsted – have around 2,500 pupils between the ages of 11 and 16.
According to the Daily Echo the letter, sent on 26 May by headteachers Andrew Portas and Annie Eagle, said: “In the last two weeks, there have been a small number of cases of students from both our communities requiring hospital treatment after using vapes.
“It is suspected that the contents of these vapes were not what they appeared to be, and this is currently being investigated.
“Students who have been affected by this have presented with accelerated heart rates far exceeding normal levels, confusion, and, in one case, fitting and loss of consciousness.”
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Vapes usually contain a flavoured nicotine fluid.
Prime Minister Rishi Sunak recently announced a campaign against underage vaping amid concerns manufacturers are targeting children in their advertising.
Mr Sunak’s crackdown comes as a new survey last month found the number of children trying vaping has risen by 50%.
The Mountbatten School was founded by Queen Elizabeth II’s cousin Lord Mountbatten in 1969. Its alumni include Downton Abbey actress Laura Carmichael and TV gardener Charlie Dimmock.
Nearby Romsey School’s past pupils include Conservative MP for Romsey and Southampton North, Caroline Nokes.
