All three cases were reported within 14 days, which health officials classify as an outbreak.
School officials said they are doing everything they can to contain the spread.
RELATED: COVID-19 cases increasing in Mariposa County
Starting Monday, parents and students will be required to complete a symptom check every time they step foot on a Mariposa County campus.
School administrators reminded parents that if their child does not feel well, they should stay home, regardless of their vaccination status.
Officials are also encouraging everyone to get vaccinated.
RELATED: CDC COVID risk map highlights areas of high transmission in Central California
For more information on vaccine or testing sites in the county, click here.