Greater Manchester Police sent firearms experts to Plant Hill high in Blackley last week, in an initiative aimed at preventing youngsters from carrying imitation guns as "fashion accessories".
It is likely to be rolled out across the city later this year after inquiries from a number of other schools.
Teresa Spooner, learning resource manager at the school, said:"Replica guns look just like the real thing, and an armed police officer is not going to stop and ask questions if he sees you with one."
Fears over gun crime in the city have been fuelled by a survey showing they are an everyday part of children's lives. The Manchester Evening News revealed that a third of schoolchildren claimed to have seen a gun and almost half said they knew someone who owned a firearm. More than one in ten of those questioned said they would consider carrying a gun.