Classes are cancelled at a Las Vegas middle school after the student body was quarantined for up to 17 hours over possible mercury exposure.

The Clark County School District said all students and staff at Walter Johnson Junior High School in western Vegas were ordered to stay on campus to be screened by the Environmental Protection Agency after mercury was discovered on site just before noon Wednesday.

Many parents notified through automated messages of the hazmat situation staked out the school until their children slowly trickled out throughout the afternoon and into the early morning hours.

It took until 5 a.m. Thursday for everyone to be screened and cleared.

Officials said Wednesday that a small amount of the element known for its toxicity was brought to campus by a student.