Many of the 69 pupils, aged about 11, were reduced to tears when they were told to hide under tables and keep quiet as a gunman was on the loose.
Parents of the children, who attended Scales Elementary school in Tennessee, were said to be furious at the "stunt".
The school spoke of "poor judgment" but did not comment on disciplinary action.
Students on the week-long school field trip had been told to expect a "campfire prank" but the mock attack went too far, local education officials said in a statement on Sunday.
'We flipped out'
The incident lasted about five minutes and was intended to be a learning experience, said the school's assistant principal, Don Bartch, who led the trip.
"We got together and discussed what we would have done in a real situation," AP news agency quoted Mr Bartch as saying.
One pupil said the lights went out and about 20 children started to cry. A teacher, wearing a hooded jumper, also pulled at a locked door.
"I was like 'Oh My God.' At first I thought I was going to die. We flipped out," said 11-year-old pupil Shay Naylor.
The move angered many of the children's parents - especially in the light of last month's massacre at Virginia Tech university, where a student killed 32 people and himself.
"The children were in that room in the dark, begging for their lives, because they thought there was someone with a gun after them," said Brandy Cole, whose son was on the trip.
The director of schools is to decide what action - if any - should be taken against the staff involved.