The police issued an urgent warning after finding "Fortnite" pins that conceal a knife.
West Midlands police said it looked like the items were sold as pens from the well-known Fortnite game.
But inside is a small blade hidden, reports Birmingham Live.
The teachers at a college were told, "Contact the security forces immediately if you see something like that."
Images of the pens were sent to the staff as a warning after the knives were seen in the West Midlands region.
On Facebook, photos and a video of the knife were published to warn parents across the country.
An officer's warning was, "Hello everyone, we've been notified of these" Fortnite "pens emerging in the West Midlands.
"If you could pass this on to your teams, and if something like that should be visible, contact a member of the security department immediately."
It came after Tarking and Dagenham Police tweeted that a 12-year-old boy in possession of one of the pens had been found.
The troupe wrote, "Please be aware of this extremely dangerous, seemingly innocent pen I just found on Facebook.
"These are sold, it seems, as pens of the infamous Fortnite Game.
"Inside, found by a Sheffield school, was a small, hidden blade."