Pueblo County High School was on lockdown for almost two hours after authorities say a student was apprehended wearing a trench coat and gas mask.
The lockdown was lifted around 10:30 a.m.
A spokesperson with the Pueblo County Sheriff's Office says the girl was not carrying any weapons, and told staff at the school that her outfit was a Halloween costume.
Other students noticed the girl and told their teacher.
"They were great," District 70 Superintendent Ed Smith said. "They reported immediately to a teacher what was happening...we could react quickly because of those students.
"That teacher reported it to administration right away, administration went to the scene, and escorted the individual to the office.
"When being questioned, the student said it was nothing more than a Halloween prank, but again, because of the world we live in now, we take all those kinds of things very seriously.”
SWAT swept the school to ensure it was safe. Members of the SWAT team were training nearby, and were able to respond quickly after receiving the call around 8:40 a.m. Wednesday.
The student is now facing expulsion.