LAS VEGAS — A Las Vegas middle school is back in session nearly a week after a mercury discovery forced more than 1,000 students into a quarantine that lasted overnight.

The Clark County School District said Tuesday that students are back at Walter Johnson Junior High School.

A few dribbles of the neurotoxin were first found on campus Wednesday morning, prompting a hazmat situation that lasted until 5 a.m. Thursday.

It was the largest decontamination effort in the history of the school district and Las Vegas fire department.

The U.S. Environmental Protection Agency says the cleanup effort has yielded more than a quarter cup of the element.

Officials say the campus has been fully decontaminated though they’re still investigating how the mercury got to the school.