The upper campus of Linwood Monroe Arts Plus school in St. Paul was placed on a brief lockdown Tuesday afternoon after a staff member overheard students talking about a weapon, according to a message to parents from principal Bryan Bass.

Bass called police to report a student might have a gun in her possession or in her locker, said Sgt. Mike Ernster, a St. Paul police spokesman. A search of the building turned up no weapons.

The lockdown began shortly before the upper campus, which houses grades 5 through 8, dismissed students for the day, said St. Paul Public Schools spokesman Kevin Burns.

A staff member had overheard a student ask another student if the gun she had was real, Ernster said. The information was conveyed to Bass, who put the school on lockdown.

After the building was declared safe, an investigation determined that the students had been discussing a Snapchat post — not from that day — that included a photo of a 12-year-old female student with a toy gun, Ernster said.

“It is important for students to understand that almost anything that is posted online can have consequences,” Bass said in his message to parents.

The campus was locked down for about 20 minutes, Burns said, delaying dismissal by about seven minutes.

Mara Gottfried contributed to this report.