Glen Ellyn police said on X about 2:20 p.m. that students at Glenbard West High School were operating under a "Secure and Teach" protocol Tuesday afternoon.
Police are investigating the call.
Officials said there had been a heavy police presence in the area.
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Police did not immediately provide any additional information about the call.
According to the school's website, eighth period typically ends around 2:20 p.m.
Police said about 2:50 p.m. that students had been released.
This is a developing story; check back with ABC7 Chicago for updates.