Authorities responded to Alhambra High School at about 7 a.m. when the smell was detected in one of the classrooms. Students who were already on the campus were evacuated.
Students were back on campus by 10:15 a.m., officials said. Police were unable to find the source of the gas smell, but they said they do not believe it was anything of significance.
Streets in the area were closed due to the hazmat investigation, including 2nd and Main streets, 2nd Street and Commonwealth Avenue, 4th and Main streets and 4th Street and Commonwealth Avenue.