Students in kindergarten through fifth grade, along with special education classes, returned to classrooms for the first time in months.
The district is responsible for educating more than 3,500 students across six campuses.
St. Norbert Catholic school in Orange also reopened on Tuesday, among a handful within the Diocese of Orange to do so.
Tuesday's hybrid schedule in Los Alamitos was far from normal; classes are three hours long split between the morning and afternoon.
Dr. Margaret Bredehoft, director of the Health Care Agency's Public Health Services, said this week the county has received 130 applications for waivers from schools to reopen kindergarten through sixth grade classes for personal instruction. The county has approved 119, and the state has approved 98 of those, Bredehoft said.
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All of them are religious or other private schools except for the ones in the Los Alamitos Unified School District.
If schools are allowed to reopen from kindergarten through high school on Sept. 22, it will be left up to each school district whether to do so.
The rate of county residents testing positive for COVID-19 was at 5% as of Friday, which is below the state's desired threshold of 8%.
The county's new case rate per 100,000 residents over seven days is 5.6. To move to the next tier, the county has to be between 4 and 7.
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The OCHCA reported that 683,364 COVID-19 tests have been conducted, including 7,555 reported Friday. There have been 43,005 documented recoveries.
City News Service contributed to this report.