WALNUT CREEK (CBS SF) — Officials at Las Lomas High School in Walnut Creek on Wednesday confirmed a recent outbreak of over 20 students testing positive for COVID-19 that appears to have originated at an off-campus, non-school event.

A statement attributed to Las Lomas principal Tiffany Benson and Acalanes Union High School District Superintendent John Nickerson stated that “21 students tested positive for COVID-19 in the last two weeks of April.”

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The release said that the origin of the cases appeared to have been “a single off-campus (and non-school) event that the students who tested positive attended or in some cases their siblings attended.”

Officials did not describe the nature of the event in the release. School officials said there did not appear to have been any on-campus transmission of COVID-19.

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“Students who tested positive were isolated at home and school staff and the District implemented a thorough contract tracing process with the support of Contra Costa Health Services,” the statement read.

Additionally, over 100 other students were directed to quarantine due to contact with students who tested positive, according to Las Lomas officials.

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Officials said all students were able to continue attending classes remotely via Zoom during their quarantine period. They noted that “isolation and aggressive contract tracing resulted in quick containment.”