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.
