Mount Pleasant Police said the woman called 911 on Thursday and said the person on the video at Wando High School was armed.
Police say they had students and teachers remain behind locked doors while they investigated.
Police said in a series of messages on Twitter that no one was hurt and no shots were fired. They have not announced any arrests.
Authorities lifted the lockdown after about an hour.
Wando High School is in the Charleston suburb of Mount Pleasant and has about 4,000 students.
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