Students and staff at Sage Canyon Elementary in Carmel Valley were nauseous and vomiting at the school Wednesday morning, according to a letter sent to parents.
Classes resumed Thursday, but Del Mar Union School District officials decided to extend the closure Friday to prevent further spread of the suspected norovirus.
"This is definitely the weirdest thing that's ever happened, I've been at the school for seven years, definitely the weirdest thing that's ever happened," said Jackson Dunn, a sixth grader at the school.
Parents were told to make sure their kids don't touch their face and keep their hands clean.
The Department of Health and Human Services are investigating the illness.
The district has hired a cleaning company to sanitize the classrooms, bathrooms and other common areas on the campus.
School will be back in session on Tuesday, May 31.