Thousands of coronavirus tests taken by children within the Chicago Public Schools system were deemed "invalid" after the tests piled up at FedEx drop boxes. 

"We don’t even know if they’re going to make it through, or if they’re going to be compromised just sitting here unattended," Chicago Public Schools parent Lorena Paredes told CBS Chicago

Data from the Chicago Public Schools website shows 35,816 tests were completed between Dec. 26 and Jan. 1, but 24,986 were deemed "invalid," the outlet reported. 

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The thousands of tests were taken ahead of children returning to class Monday following holiday break. Tests must be processed within 48 hours. 

The CDC said that home tests must be supervised by a telehealth provider associated with the test manufacturer.

The CDC said that home tests must be supervised by a telehealth provider associated with the test manufacturer. (iStock)

The testing vendor, COLOR, told parents that the tests were delayed because of "weather and holiday-related shipping issues." The vendor told CBS Chicago it’s supporting extended testing drop-off hours and more testing this week. 

"When we saw the photos last week of tests stacked up outside of drop boxes, right there, that was a signal that we had a problem," Mercedes Carnethon, vice chair of preventive medicine at the Northwestern University Feinberg School of Medicine, told CBS Chicago. 

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FedEx said in a statement to Fox News on Tuesday that it "continues to work closely with customers on the movement of critical COVID-19-related shipments such as test samples, kits, vaccines, and supplies as COVID-19 cases spike throughout the U.S. The safe and secure transportation of these shipments remains our top priority."

"We moved swiftly and flexed our network in order to accommodate the sudden influx of test sample volume from Chicago Public Schools, collecting and shipping over 35,000 test kits within the specified timeframe," the statement continued. 

CPS encouraged all students to get tested ahead of returning to class, but it was not mandatory. Parents say that students who do not appear in the classroom are marked absent, according to WGN9

CPS’ media relations team did not immediately respond to Fox News’ request for comment on how many students have missed class this week due to the invalid tests. 

A sign about COVID-19 testing is displayed outside of COVID-19 testing site as a health walker collects testing tube in Wheeling, Ill., Friday, Dec. 3, 2021. A week after Thanksgiving, Illinois on Thursday reported this year's highest daily total of new coronavirus cases, while COVID-19 hospitalizations have risen higher than any point since last winter. (AP Photo/Nam Y. Huh)

A sign about COVID-19 testing is displayed outside of COVID-19 testing site as a health walker collects testing tube in Wheeling, Ill., Friday, Dec. 3, 2021. A week after Thanksgiving, Illinois on Thursday reported this year's highest daily total of new coronavirus cases, while COVID-19 hospitalizations have risen higher than any point since last winter. (AP Photo/Nam Y. Huh)  (AP)

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"What a nightmare," Carnethon said. "The strategy of using a mail-in test during a busy time of year for the Postal Service, and when weather delays happen, was a faulty strategy from the start. And there was a large risk of what we saw happen today, which is that many children returned to the classrooms and we don’t know their status."

The Chicago Teachers Union is expected to hold a vote later Tuesday on remote learning due to the surge in cases, which has sparked outrage from some parents. 

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"We're simply relying on public health guidance and from every level of the public health experts out there they say in-person school is safe and should be prioritized, but well, that's never good enough for the Chicago Teachers Union," Chicago father Ryan Griffin told "Fox & Friends" on Tuesday.

"They will never pass up an opportunity to exert their leverage, and they'll get power plays, and unfortunately, our kids here in Chicago are the ones paying the consequences," he continued.