LAS VEGAS – Students at a Las Vegas junior high school were quarantined into the night Wednesday to test for mercury poisoning after a student brought the toxic substance to school.

Frustrated parents spoke out about the Clark County School District’s lack of communication about the situation as they waited to pick their children up from Walter Johnson Junior High School, where many were held by the Environmental Protection Agency well into the night, KSNV reports.

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Las Vegas Fire and Rescue told the news site that officials responded around 11:40 a.m. after a student brought a “very small amount” of the liquid to school Wednesday, and five or six students were exposed to the substance. The parents of those students were notified immediately, but the hundreds of others who gathered in front of the school to pick up their kids were left to worry about their safety.

“There are 500 people out here and instead of hearing something about it – you hear your kids are fine, they are just getting checked out – we didn’t get any of that,” parent Anthony Flores said.

“To stand 100 yards away from the entry to this school and not be able to see him, touch him and at least give me a visual that he’s okay, he’s fine, he’s walking and has all his clothes and shoes, just make sure he is not contaminated, they won’t even give me that,” he said.

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Las Vegas Now reports the school’s gym was closed off but classes resumed for most students as officials with the EPA assessed the situation and performed required health checks on students.

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Students were held at the school during the screening, which lasted until 5 a.m. Thursday before the quarantine was lifted. The EPA confiscated students’ shoes and backpacks for testing, as well.

District officials canceled classes at the school for Thursday, a precautionary measure “to ensure that the school can be cleaned before students return,” according to a prepared statement.

“Johnson Middle School had a chemical element mercury contamination on campus today. We believe the mercury was brought onto campus by a student. According to EPA guidelines, all students must be tested before they can go home. Safety is our top priority and we want to make sure students are safe to leave the school,” the statement read.

“No one is in immediate medical danger, the goal is to contain exposure. Food will be provided to students via CCSD’s food service department. When your child has been cleared he or she will call you so that you can pick your student up.”

At 8 p.m., hundreds of students were still waiting to undergo the EPA’s screening process, according to The Las Vegas Sun.

“Officials said late Wednesday that additional support staff was brought in to help with the EPA screening and that students were being allowed to eat as they wait,” the news site reports.

Several parents told Las Vegas Now that the process was a frustrating ordeal.

“I feel like in this situation, you know, if your parent comes to get you, you should be able to go,” parent Jamie Garrett said.

Parent Bernard Conrique also complained about “the lack of information.”

“They’re not saying this, and they’re not saying that,” he said, “or they’re probably sugar coating what’s going on.”