Blaine County School District spokeswoman Heather Crocker says someone fired a shotgun through a glass door on the north side of the school to gain access. She says they were in the building for just over a minute, pulled the fire alarm and left.
District officials were notified that an alarm was activated at the school at about 1:30 a.m.
Police inspected the entire high school and cleared it for occupancy, delaying the start of classes until 10:30 a.m. The district said there would be an increased police presence at the school for the rest of the day.
The break-in is still under investigation.