Updated 08/27/13 – 2:21 p.m.

CHICAGO (CBS) — Students at a Grand Boulevard neighborhood elementary school were being bused to another school, or allowed to go home, after a transformer failure caused a power outage.

The power went out around 11 a.m. at Mollison Elementary School, at 4415 S. King Dr., according to ComEd and the Chicago Fire Department.

Chicago Public Schools spokeswoman Becky Carroll said the transformer is located outside the building, so “no students were nearby when the incident occurred and all students at Mollison are safe.”

Parents were notified of the outage by email and robo-call, Carroll said.

A transformer failure set off the school’s fire alarm, prompting officials to evacuate the 600 students from the building. When firefighters discovered there was no fire, students and staff were allowed back inside, and given bottled water to cool of.

“I heard the explosion, actually, because we live half a block away,” said Ebony Tillman, after coming to Mollison to pick up her 7th grade daughter, who thought the alarm was just a fire drill.

“We was just sitting in the class doing our work, and we just heard a boom, but we didn’t know what it was, and we kept finishing our work, and then when the alarm went off, we thought it was a fire drill at first, but we just went out the class like we were supposed to,” Jumoke Tillman said.

The outage happened on the second day of school, and on one of the hottest days of the year. At 11 a.m., it was 88 degrees in Chicago, with a heat index of 94.

Due to the power outage, parents were allowed to take their kids home. Students who were not picked up were bused to nearby King College Prep High School “to receive lunch and be placed into a safe and comfortable environment with their teachers,” according to Carroll.

Students who were not picked up by their parents already would be bussed back to Mollison in time for its normal 3:45 p.m. dismissal, so their parents can pick them up then.

Mollison is one of the Chicago Public Schools’ so-called “welcoming schools,” taking kids from Overton Elementary, which was closed this summer.

Parent and local school council member Jeanette Taylor said Mollison is still under construction, and she believes it wasn’t ready for students when classes began this week.

“They have nothing figured out. Anywhere in the history of them closing schools, when have they ever got it right? Never,” she said. “If you closed 50 schools back in June, this building should have been ready. It’s only because of the kids you’re serving. … If this was up north, this building would be ready. This is not right, and they know it. And I don’t care that no children or staff got hurt, it’s still wrong.”

However, Carroll said there was no construction at or around the school at the time. She also said it’s not uncommon for transformers to fail in such heat.

ComEd spokeswoman Ashley Dennison said the cause of the power outage was under investigation, but she said some of the fuses on the transformer blew out. She said repairs would take 6 to 8 hours.

CPS said a generator would be brought in to make sure the school has power and can open on time on Wednesday.