It happened at Vintage High School in Napa.
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The morning after, it was business as usual at Vintage High School as students returned to class.
The Napa Valley Unified School District says many of the affected students were in their first-period choir class at the time.
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"She said that a couple students had passed out and one was throwing up and everyone just came inside and started getting people out and putting them into a different room to test everyone," Elyana Calderon, a Vintage High School student said.
Calderon is a senior at the school who spoke with one of the students who was taken to the hospital.
She says about halfway through first period all students were told to shelter in place, which lasted for hours.
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"We heard that there was like a SWAT team, police and firefighters were there and so everyone was just kind of chaotic really and people were calling their parents wanting to go home because we didn't know what was going on," she said.
A spokesperson for the district says the school is in session on Friday after fire officials deemed the building safe.
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PG&E crews found no gas or carbon monoxide leaks.
"The fire department, police department and PG&E have deemed not only the choir room but the band room that is adjacent to it and the entire building as safe for use," Julie Bordes, a spokesperson for the Napa Valley Unified School District said.
The district says the hospitalized students were back home recovering by end of the day.
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"Our number one priority is the health and safety of our students, that always comes first," Bordes said. "We feel confident with the guidance of our emergency responders that our students can be on campus and safe."
As students at Vintage High School look forward to a three-day weekend, the cause of what made around two dozen students sick Thursday is still under investigation.