Students at Arlington High School were once again evacuated Thursday afternoon due to an odor of natural gas.

The first incident happened on Monday, when a teacher smelled gas after school, requiring students at after-school programs to leave the building. On Thursday, students were evacuated school about 1:15 p.m. after a teacher smelled a strong odor of gas on the school’s fourth floor, by the science classrooms.

Senior Joshua Ghofrani, 18, said was at his English class when an alarm went off. He said students were out for about 20 minutes before being told to return to class.

“They said if you don’t have a sixth period, you should just leave the school,” Ghofrani added.

Students were allowed back in the school about 1:35 p.m., an official said.

National Grid crews were at the school after the incident.

During both incidents, school officials said there is no danger to students or staff.