Following a Thursday evening evacuation due to a toxic substance found in a school bathroom, classes at Lane Tech College Prep High School resumed as usual Friday, with a hazardous materials cleanup planned to continue in restricted areas over the course of the weekend.

Principal Edwina Thompson alerted students’ families with a pair of messages Thursday on the “small quantity of mercury” found and the response by the Chicago Public Schools Department of Facilities and a hazardous materials team, which prompted the school’s evening closure and cancellation of events.

Of the nearly 4,500 total students enrolled at Lane Tech, some were still on campus Thursday evening, the student newspaper the Lane Tech Champion reported, with the incident disrupting a girls basketball game, whose referee evacuated students from the building. A senior student told the Champion they’d observed “liquid metal” in a toilet Thursday afternoon and emailed the principal after conferring with one of their teachers.

In a follow-up statement sent to Lane families late Thursday evening, Thompson wrote the response team “did not find widespread elevated levels of mercury” in a school-wide inspection. Thompson said classes would resume as normal Friday, though “several areas of the school” would remain off-limits out of an abundance of caution. “These rooms will be cleaned over the weekend to ensure they can be safely reoccupied as soon as possible.”

According to the Centers for Disease Control, the potential effects of exposure to elemental mercury, which is liquid at room temperature, depend on the concentration and duration of exposure. “At low vapor concentrations over a long time, neurological disturbances, memory problems, skin rash, and kidney abnormalities may occur,” the CDC states. “Very high mercury vapor concentrations can quickly cause severe lung damage.” Exposure to other forms of mercury can occur in workplaces where mercury compounds are used, such as industrial operations, and through eating contaminated fish.

In response to a Friday morning request for details on the mercury found and length of time it had been present, Chicago Public School referred to Thompson’s initial statement without providing additional information.

The 47th Ward Democrats are due to host a forum of the Chicago mayoral candidates at the school Sunday evening.