Haddam-Killingworth High School in Higganum has been evacuated for the second time in two days because of something in a chemistry classroom.
Crews were at the school Wednesday to clean up after a thermometer broke Tuesday when they found another substance in the room.
The item was not properly labeled and officials were not sure of the makeup of the substance, so they evacuated the school as a precaution, police said.
The students were moved to the middle school as the Department of Environmental Protection assesses the situation.
No one was injured and no students had anything to do with the discovery of the substance, police said.
The high school was also evacuated last week for another chemistry lab incident. Carbon disulfide caused a chemical reaction and small flame.