Police were called to Kingussie High School after the parent of one teenage boy found a twelve-bore shotgun cartridge in his schoolbag.
Seven other pupils, all aged between 12 and 14, have also been questioned.
A spokesman for Northern Constabulary stressed that no gun had been involved.
Support officer Norman MacLeod, who runs the force's firearms licensing department, said that it was not against the law to be in possession of shotgun cartridges, unlike ammunition for class 1 firearms such as rifles and handguns.
He added that the person who left the shotgun ammunition in such a way to allow children access, had not committed an offence.
He said: "It is an offence to sell shotgun cartridges to a person who doesn't have the appropriate gun certificate, but not to possess them as such.
"It is only a guideline that shotgun cartridges should be kept secure, but we would always be wanting people like gamekeepers and farmers to keep shotgun ammunition out of the wrong hands.
"Leaving it insecure would put a question-mark over whether the person was suitable to have a shotgun certificate in the first place."
Firearms expert
Force firearms expert Sgt Graham Askew said: "Shotgun cartridges are relatively safe, and the primer would have to be struck or exposed to a fair bit of heat before they would go off.
"But there have been cases of youngsters hitting cartridges and rifle rounds with hammers and a nail. Such ammunition is best kept away from children."
The latest suspensions follow a drugs probe at the school in January when six pupils admitted smoking cannabis in Aviemore.
Three pupils were suspended after parents informed the school and police of that incident.
Kingussie rector Dr Tom Taylor said: "Eight pupils have been excluded for having shotgun cartridges at school.
Pyrotechnics display
"The matter came to light when police followed up a parent's report of finding a cartridge at home.
"While there was clearly no malicious intent, and those involved are decent young people, these days of course we have to take a very firm line regarding such things in school where they could, to say the least, blind or maim a child.
"I'm very happy to say that the parents of the children involved in the matter view the matter equally seriously and are, in general, working with us in a very responsible and supportive way."
The children are believed to have been cutting the end of the cartridges and pouring out the gunpowder to use for pyrotechnics displays, but it is also understood this did not happen at the school.
A police spokesman said that no-one had been charged or arrested in connection with the incident and that the investigation was concentrating on where the ammunition had come from.
He said: "Inquiries at the moment are continuing."