Two Oundle School fifth form students have been expelled after being caught inhaling laughing gas.
Five further fifth form pupils will leave the school by the end of the academic year.
"The incident involved the use of nitrous oxide, which, whilst having legitimate culinary and medical uses, is forbidden within the school rules and under the school’s illegal drugs and illicit substances policy," said a spokeswoman.
Oundle School, which charges £36,960 per year for third form to sixth form boarders and £23,505 per year for third form to sixth form day pupils, has a zero-tolerance policy on illegal or illicit drugs.
Headteacher Sarah Kerr-Dineen said: "Psychoactive substances, commonly known as legal highs, are of widespread concern across the country.
"As a school, we seek to support pupils in understanding the danger of drugs of all natures and to engage in informed discussion so that they can make good decisions for their own welfare and for those of others.
"Our policy on illegal and illicit substances in school is one of zero tolerance and those in contravention of this can expect to be asked to leave.
"It would not be appropriate for the school to make any further comment on this individual case.”